The period during which a developing fetus is carried within the uterus. In humans, pregnancy averages 266 days (38 weeks) from conception to childbirth. Traditionally, pregnancy duration is counted from the woman's last menstrual period, which adds roughly 2 weeks to gestational age. This is how physicians arrive at a pregnancy length of 40 weeks (280 days).
The 9 months of pregnancy are typically divided into three periods (trimesters) of 3 months. The first sign of pregnancy is often the absence of an expected menstrual period. Common symptoms include nausea, breast tenderness, fatigue, and frequent urination. The diagnosis of pregnancy can be made as early as 10 days after fertilization by means of blood tests. By 6 weeks (from the last menstrual period), the uterus feels soft and is palpably enlarged. Pregnancy can be positively confirmed by observing cardiac motion of the fetus by ultrasound scanning (8 weeks) or by hearing fetal heart “tones” by using a Doppler detection instrument (10–12 weeks).
Early in the first trimester, the embryo's germ layers differentiate into organs and systems, a process that is nearly completed by the twelfth week. It is during this critical period of development that the fetus is most vulnerable to the adverse effects of drugs and other teratogenic influences. The second and third trimesters of pregnancy are characterized by increased fetal growth and gradual physiologic maturation of fetal organ systems. During this time, the maternal changes of pregnancy are greatest. The enlarging uterus encroaches on the abdominal region by the fourth month and at term nearly reaches the diaphragm. The breasts gradually enlarge in preparation for lactation. Striking cardiovascular changes, including nearly a 50% increase in cardiac output, provide the increased blood flow to accommodate the growing fetoplacental unit. Other changes in the renal, digestive, pulmonary, and endocrine systems reflect the numerous maternal adaptations that eventually must occur in a healthy pregnancy. See also Embryology.
Early, regular prenatal care is associated with improved pregnancy outcome and seeks to identify risk factors in the pregnancy that may apply to mother or fetus. At 6–8 weeks, a complete physical examination, along with blood and urine analyses, should be performed. In addition to undergoing traditional tests, patients are now routinely screened for hepatitis B at the beginning of pregnancy, for fetal neural-tube defects such as open spine at 16 weeks, and for gestational diabetes at about 28 weeks. In addition to these blood tests, many physicians offer a sonogram at 16–18 weeks to establish gestational age, check for a multiple pregnancy, and screen for birth defects. During prenatal visits, a physician can evaluate nutrition, blood pressure, and fetal growth. See also Prenatal diagnosis.
Ideally, at the end of the third trimester, the process of labor begins. The muscles of the uterus contract, dilating the cervix and allowing the baby to begin moving into the vagina or birth canal. Continued contractions push the baby out of the mother's body. In the final stage of labor, the placenta detaches from the uterine walls and is expelled as the afterbirth. An alternative to vaginal delivery is the cesarean section, in which the baby is removed surgically through an abdominal incision.
The legal status of pregnancy termination (therapeutic abortion) varies from country to country, but about two-thirds of women in the world have access to legal abortion. Over 90% of abortions in the United States are performed in the first trimester by suction curettage, a technique that uses suctioning and removal of the uterine contents through the vagina with surgical instruments. Later pregnancies are terminated by a procedure called dilatation and evacuation (D&E) or by administration of drugs to stimulate uterine contractions. Medical and psychological sequelae to abortion are few, and are fewest for terminations in the first trimester. See also Pregnancy disorders.
Although women with a history of poor health may be prescribed rest at various stages of pregnancy, it is now generally accepted that moderate exercise can be beneficial. The reported benefits include:
• maintenance of physical fitness
• avoidance of excessive weight increase
• decreased risk of problems such as constipation, backache, and varicose veins
• improved sleep
• improved self-image and less risk of postnatal depression
• easier labour.
An exercise programme for pregnant mothers should take into consideration physiological and anatomical changes. The mother's heart, lungs, and other vital organs are working much harder than usual, and hormonal changes make some joints less stable. Pregnancy is not the time to start a fitness programme, but most regular exercisers can continue their normal exercise programme, with their doctor's permission.
Several elite athletes have continued to train and compete successfully during the early stages of pregnancy. Three gold medallists in the 1956 Melbourne Olympics were pregnant when they competed. During the later stages of pregnancy (usually after the 5th month) strenuous physical activity is not recommended.
Exercises which are particularly beneficial during pregnancy should be learned in special, medically-approved antenatal classes. These include exercises to improve posture and general mobility; pelvic floor exercises to maintain the perineal muscles and reduce the risk of stress incontinence in later years; and relaxation exercises, to improve neuro-muscular control and the ability to cope with the pain of labour. Many experts advise against excessive stretching because the hormone relaxin is present in the body during pregnancy. Relaxin causes ligaments to relax, particularly those in the pelvis, which needs to widen during birth. Lax ligaments mean that joints can be overstretched easily, causing long-term problems of joint instability. During pregnancy the rectus abdominis (one of the stomach muscles) separates to accommodate an enlarged uterus. Sit-ups, or other strenuous abdominal exercises, can exaggerate the separation and result in an enlarged abdomen after giving birth. The American College of Obstetricians and Gynaecologists' guidelines for exercise during pregnancy state that pregnant mothers should be especially careful to take plenty of fluids before and during exercise to limit the risk of dehydration and assist cooling (maternal body temperature should not exceed 38°C).
During pregnancy, the mother is eating for both herself and her baby. At least in the later stages of pregnancy, she needs to consume more energy than usual, but this does not mean eating twice as much. The average weight gained during pregnancy is about 10-12.5 kg (22-28 lb). There is no merit in putting on less than that, but there are disadvantages in gaining much more. Carrying excess weight is tiring. It can also increase the risk of problems such as backache, diabetes, and varicose veins. On average, pregnancy demands 200 additional Calories daily. In the UK, the Health Education Authority recommend that these extra calories should be taken only during the last three months of pregnancy. It is only then that the energy cost of providing for the baby is high enough to necessitate a greater food intake, but this assumes that activity levels are reduced during pregnancy.
Pregnancy also increases nutrient requirements. It is important that the mother's diet contains sufficient protein, iron, calcium, folate, and vitamins C and D for the formation of the baby's muscles, bones, and teeth, and to make haemoglobin. Most extra nutrients are obtained simply by eating a balanced diet that satisfies the increased energy requirements. However, all women should take folate supplements to reduce the risk of neural defects in their babies (see folic acid). Pregnant women are advised not to eat liver or liver products although they are rich in folate, because of the possible harmful effects of their high vitamin A content on the health of the baby. Supplementary iron and vitamins C and D are often recommended, but iron tablets can cause constipation and other distressing effects in some people. Any supplements should be taken only after consultation with a medicallyqualified person or dietitian.
The biological event of pregnancy is established when a fertilized egg successfully implants itself in the lining of the uterus, about a week after conception.
The corpus luteum, which formed in the ovary when it released the egg, secretes hormones that keep the uterine lining in a suitable state for implantation; if fertilization had not occurred, this hormone secretion would have ceased, and the uterine lining would be shed after two weeks. The hormonal ‘message’ from an implanted embryo via the mother's bloodstream to the ovary prevents its own rejection.
Early pregnancy continues to be maintained by the hormones produced by the corpus luteum in the ovary that produced the egg; but later, when the placenta has fully developed (by about 3 months), this takes over the maintenance function through its own hormone production.
Pregnancy produces profound changes in the mother, which may be detected from early stages. There is a marked rise in the output of the heart by 3 months, and it rises further as pregnancy advances, reaching 30-40% above the non-pregnant level by the end. This rise is mainly due to an increase in output with each contraction of the heart muscle (stroke volume), although the heart rate also increases. The volume of blood in the circulation also increases, with a greater increase in plasma volume than in red blood cells, producing the so-called ‘physiological anaemia of pregnancy’. Although these changes in the circulation can produce serious consequences for pregnant women with certain types of heart disease, they are necessary to deal with the demands of the growing fetus, placenta, and uterus, and have no deleterious effects in healthy mothers.
There are changes in the breasts from an early stage of pregnancy; they enlarge, and surface blood vessels become prominent, reflecting preparation for eventual lactation. Hormonal changes cause development of the glandular tissue: the potential milk-secreting cells and the ducts to the nipples. Although the hormones which cause milk production (prolactins) are produced during pregnancy, the actual secretion of milk is suppressed by other hormones until after delivery.
Other changes include a laxity of the joints, which ultimately may assist labour and birth, and increased brown pigmentation of the skin (‘chloasma’ if in the face). Stretch marks are other hallmarks of pregnancy in the skin. The mother has increased blood flow to the kidneys, and therefore increased urine production, and this results in more frequent visits to the toilet — a common symptom of early pregnancy. The placenta produces large amounts of the hormone progesterone, which appropriately prevents the uterine smooth muscle from contracting, but also relaxes smooth muscle throughout the body. This results in many of the so-called minor symptoms of pregnancy, including constipation and heartburn, and it may exacerbate varicose veins.
The mother's appetite usually increases — but the extra energy requirement for the whole pregnancy is not more than about 60 000 Kcal — or 20-24 extra days' worth of food intake. Where there is abundance of food, excessive eating and undue weight gain are not uncommon, although there is in fact a normal physiological tendency to lay down more fat stores in the earlier months. Appetite for particular foods and drinks, or rejection of others, can be capricious. Occasionally the nausea of morning sickness, which is common in early pregnancy, may extend to other times of day, may be more severe than usual, and may be accompanied by vomiting or may be prolonged into later pregnancy.
The uterus enlarges considerably to accommodate the growing fetus. It emerges from the pelvis at around 12 weeks, reaches the navel at around 22 weeks, and the ribs at around 36 weeks.
Pregnancy normally reaches its dramatic conclusion with the onset of labour, between 35 and 39 weeks after conception.
The establishment of antenatal care to detect problems during pregnancy, and to attempt to ensure that women were in good health at the time of delivery, is generally credited to J. W. Ballantyne, an Edinburgh obstetrician, who took the first step towards this at the beginning of the twentieth century. Clinics became established in major centres in the UK, the US, and Australia by the time of the first World War.
— Jim Neilson
Pregnancy: the cultural context
Pregnancy occupies potent symbolic space in cultures around the world. As both the development of a life and a significant transitional event within the woman's lifespan, pregnancy becomes the focus of cultural desires and anxieties around gender, power, selfhood, and even nationhood. Medical technology has increasingly refigured the physiological possibilities of pregnancy, especially through assisted reproduction for the infertile, its extensions to surrogacy and older-age pregnancy, and through genetic testing.
One of the most common cultural mythologies about pregnancy is that it is evidence of full womanhood. Because mothering is so closely tied into cultural gender roles, to be pregnant is to fulfill one's gendered destiny. Although this emphasis on pregnancy emerges from culturally-specific definitions of femininity and womanhood, many people see the urge as instinctive and the process itself as natural, even as industrialized countries increasingly rely on medical technologies to avoid, create, sustain, and complete pregnancies.
Differential worldwide rates of fertility, infant mortality, and maternal mortality have led the World Health Organization to focus attention on women's differential access to services and opportunities with respect to men as well as between different countries and regions. At least partly because of this focus, all three of these rates dropped by about one-third over the twenty years up to 1998, when overall fertility rate was 2.7 births per woman; Europe was lowest at 1.6, while Africa remained highest at 5.4. Infant mortality rate world-wide was 57 deaths per 1000 live births, whereas highly industrialized countries such as the US and the UK had rates as low as 7 deaths per 1000. Maternal mortality rate (expressed as deaths per 100 000 births) in the UK showed a dramatic drop from the 1930s onwards, whereas until then it had been essentially unchanged at around 500 for 100 years; in the 1980s it was below 10. By the end of the twentieth century, according to the World Health Organization, developed nations averaged a rate of 27 deaths per 100 000 live births. This contrasts with 480 on average in developing nations (comparable to Victorian Britain), with some regions as high as 1000. The global average was 430. While these numbers are specific to pregnancy, and associated with disparities in medical services and supplies, they may also reflect the status of girls and women in different cultures, and their relative power in their societies.
Pregnancy, in the natural order of things, becomes possible and physiologically appropriate as soon as ovulation is established after the menarche, usually during the teens, or even earlier. But in modern developed societies, the issue of teenage pregnancy is increasingly a concern to both moral leaders and health educators. In the UK the rate has been rising: in 1997, under-16s accounted for over 8% of all known conceptions in the under-20 age group; meanwhile rates declined in other European countries and in the US there has been some reduction since the late 1980s. The spectre of the pregnant young girl is often cited as a wake-up call for issues as diverse as promiscuity, health education, and the viability of the welfare state.
Young women who maintain pregnancies are less likely to finish or continue their education, face greater marital instability, have fewer lifelong assets, and have lower incomes later in life than women who did not become pregnant young. Yet pregnant teenagers have become symbolic more of the decline of social morality than of the lack of resources granted to young women worldwide.
Teenage and unmarried pregnancies have always existed, but the advent of new methods of contraception in the twentieth century has changed the significance and experience of pregnancy for hundreds of millions of women worldwide. Before these methods were widely and legally available, pregnancy often signified the end of a woman's career choices, if not her need to work; closely successive pregnancies, when timing could not be controlled, often led to early death, as it still does in many places worldwide today.
Female-directed methods, such as the modern intrauterine device (IUD) and hormonal control by the Pill or by long-lasting implants, have allowed women to choose not only the occurrence but also the timing of pregnancy. Earlier barrier methods of contraception had allowed women to control their pregnancies somewhat, although they also required them to negotiate with their husbands. Hormonal contraceptives have changed many women's relationship to pregnancy by putting the choice in their own hands. Indeed, world health leaders are calling for this globally as a step towards women's liberation from socially imposed controls.
— Julie Vedder
A great deal of public health resources is spent on pregnancy. It is clear that prenatal and neonatal health play a large role in determining the health of a population, and in fact, pregnancy outcomes are often used as an indicator of a nation's health.
Epidemiology of Pregnancy
Globally, there are approximately 240 million pregnancies annually. These pregnancies result in 134 million births and 50 million induced abortions, 20 million of which are performed under unsafe conditions. Approximately 6 to 7 million of these 240 million pregnancies occur each year in the United States. These result in about 4 million liveborn babies, over 1 million induced abortions, at least 1 million spontaneous abortions (miscarriages), nearly 100,000 ectopic pregnancies (a pregnancy in which the fetus develops outside the uterus), and about 30,000 fetal deaths.
Of the 4 million babies born in the United States in 1999, 12 percent were born to women under 20 years of age. Approximately 4.5 percent of white teens (ages 15 to 19), 8.1 percent of African-American teens, and 9.3 percent of Hispanic teens gave birth. Since 1991, the teenage birth rate has been declining in the United States, particularly among African Americans, largely because of an increased use of effective contraception.
In 1999, 13 percent of the babies born in the United States were born to women 35 years old and older. The birth rate among this age group increased during the last three decades of the twentieth century, despite the fact that older women have an increased risk for having babies with chromosomal abnormalities (the risk is approximately 1 in 1,000 at age 25, 1 in 200 at age 35, and 1 in 20 at age 45).
About half of all pregnancies are unintended or unplanned, and one in three babies are born to single or unmarried mothers. (Nearly 70% of African-American babies and over 40% of Hispanic babies are born to unmarried mothers.)
Four out of five women who gave birth in 1999 started prenatal care in the first trimester, though this percentage was lower among African-American and Hispanic women. Despite an overall improvement in prenatal care utilization, the proportion of low birthweight (LBW) births and preterm births have been increasing gradually since the mid-1980s. This increase is accounted for, in part, an increase in multiple gestations and the growing number of infants born to women older than 35 years of age.
Of the 1.2 million legal induced abortions performed in 1999, 20 percent were obtained by women less than 20 years old, 60 percent by white women, and 80 percent by unmarried women.
Physiology of Pregnancy
A human pregnancy starts when the male sperm fertilizes the ovum (egg) in a woman's Fallopian tube, and it lasts, on average, 266 days. Contraception works by inhibiting the release of the ovum from the ovary (birth control pill, injectible, or subdermal implant), by impeding the release of sperm (vasectomy), by blocking sperm from entering the vagina or cervix (male or female condom, diaphragm, or cervical cap), or by blocking the Fallopian tubes (tubal ligation). Once conception takes place, the fertilized egg travels through the Fallopian tube into the uterus, where it implants about seven days later. The intrauterine device (IUD) impedes such implantation, and medications like mifepristone (RU486) causes the implanted embryo to abort.
A developing human is called an embryo between two and eight weeks after conception; thereafter it is called a fetus until delivery. Development of the major organs begins during the early embryonic period, and interference with this process may result in birth defects. Women taking harmful substances, or women with preexisting diseases like diabetes mellitus, are at increased risk for having babies with birth defects. Because the development of major organs begins during early pregnancy, often before a woman starts prenatal care or realizes that she is pregnant, preconceptional care is recommended for every woman of reproductive age.
Although most major organs are present at the end of the embryonic period, the development of their functions continues well into the fetal period, infancy, and early childhood. Interference with this process may lead to functional deficits. For example, undernutrition during this period of growth has been associated with increased risk for coronary heart disease, and maternal alcohol use during pregnancy has been linked to mental retardation and other birth defects.
Remarkable changes take place in a woman during pregnancy. The heart circulates 40 percent more blood volume to supply nutrients and oxygen to the growing baby, deeper breaths occur and an increased amount of harmful substances are cleansed through the kidneys. Digestion slows down for better absorption or nutrients, which may cause problems such as heartburn and constipation. The baby is sustained in the uterus by the placenta, which serves as the interface between maternal and fetal circulations. Hormones prepare the breasts for lactation, and the immune system is altered so that it does not reject the baby as a foreign body. While most healthy women make these adaptations readily, pregnancy can jeopardize the health, and sometimes the lives, of women who are less healthy and suffer increased stress to the system during pregnancy.
Pathophysiology of Pregnancy
When things go wrong during pregnancy, the health of both mother and baby may be at risk of certain health problems associated with pregnancy.
Infertility. Infertility is defined as failure to conceive following a period of 12 months or longer of unprotected sexual intercourse. In 1988, over 8 million American women 15 to 44 years of age reported an impaired ability to have children. Major causes include endometriosis, poor sperm quality or low sperm count, failure to ovulate, and tubal damage.
Ectopic Pregnancy. An ectopic pregnancy is a pregnancy that has implanted outside of the uterus, most commonly in the Fallopian tubes, which may have been scarred from a previous infection, ectopic pregnancy, or tubal ligation. The growing pregnancy, if not surgically terminated, may rupture the tube, causing hemorrhage. Ectopic pregnancy is a leading cause of maternal deaths among African-American women.
Abortion. Abortion refers to the termination of pregnancy before the twentieth week of gestation (counting from the last menstrual period). Abortion can be spontaneous or induced. Most spontaneous abortions (miscarriages) involve some chromosomal abnormalities; the causes of the rest are not known, but some may be due to exposure to environmental toxins.
Birth Defects. Birth defects are the leading cause of infant death and the fifth leading cause of potential years of life lost. About 3.6 percent of all babies in the United States are born with major birth defects, the most common being cleft lip and palate, Down syndrome, neural tube defect, and congenital heart disease.
Low Birth Weight (LBW). LBW, defined as birth weight under 2,500 grams (5.5 pounds), is the second leading cause of infant death, and the leading cause of infant death among African Americans. Risk factors include short interpregnancy interval, low prepregnancy weight, inadequate weight gain during pregnancy, history of LBW or preterm birth, cigarette smoking, and socioeconomic factors.
Preterm Birth. Preterm birth, defined as delivery before 37 weeks of gestation, may result in major problems, including neurological damage from brain hemorrhage or respiratory distress from immature lungs.
Fetal Death. Fetal death refers to the death of a fetus after 20 weeks of gestation. Major causes include preexisting maternal conditions like diabetes mellitus or hypertension, and premature separation of the placenta from the uterus (placental abruption) as a result of drug use or trauma.
Infant Death. Infant death refers to death of a baby under one year of age. Major causes include birth defects, LBW, and sudden infant death syndrome (SIDS).
Maternal Death. Maternal death is defined as the death of a woman as a result of her pregnancy, from the first stages of gestation to within 42 days after the pregnancy has terminated. Risk factors include age greater than 35, unmarried status (owing to socioeconomic factors, including a lack of access to health care), and lack of prenatal care. The classic HIT triad (hemorrhage, infection, and toxemia or preelcampsia) contributes to about half of all maternal deaths. Approximately 300 women in the United States and 500,000 women in the world die every year from pregnancy-related causes. The maternal mortality ratio of 7.5 deaths per 100,000 live births in the United States did not changed significantly during the last 20 years of the twentieth century.
Preeclampsia. Preeclampsia, caused by high blood pressure during the latter part of pregnancy, is characterized by hypertension, protein in the urine, edema, and organ damage as a result of hypertension. Such organ damage may include seizure, stroke, kidney failure, liver damage, and fluid in the lungs. Preeclampsia is treated by effecting prompt delivery (and thereby ridding the body of the circulating toxin released by the placenta). Magnesium is commonly used to prevent seizure. Complications of severe preeclampsia can often be prevented with early diagnosis and appropriate treatment.
Obstetrical Hemorrhage. Obstetrical hemorrhage is characterized by excessive blood loss. It occurs prenatally as a result of premature separation (placental abruption) or abnormal location (placenta previa) of the placenta. It can also occur as a result of injury to the birth canal during delivery, retained placenta within the uterus after delivery, or the inability of the uterus to firm up (uterine atony) after delivery.
Puerperal Infection. Puerperal infections are those that occur during labor, delivery, or the postpartum period. The infection is typically caused by bacteria from the vagina ascending into the uterus. Risk factors include cesarean section, prolonged time from when the "water breaks" to delivery, poor nutrition, and maternal anemia. Prompt treatment with antibiotics can prevent significant morbidity associated with puerperal infections.
Embolism. An embolus is a clot. It could be a blood clot (thromboembolus), or a clot of fetal tissues (amniotic fluid embolus) that travels in maternal circulation. If it blocks off circulation in the lungs or the heart, the embolus could be fatal.
Between 1900 and 2000, infant mortality in the United States declined by 90 percent, and maternal mortality by 99 percent. This was one of the greatest achievements of public health in the twentieth century. However, the goal, established in 1994 by the International Conference on Population and Development, of every pregnancy being healthy has not been achieved. Current efforts to ensure healthy pregnancy work at three different levels of prevention.
Primary prevention involves efforts to prevent diseases from occurring during pregnancy. Examples of primary prevention during pregnancy include family planning, preconceptional care, and health promotion during prenatal care. By preventing unintended pregnancies, family planning can prevent morbidity associated with unintended pregnancies. Preconceptional care has been shown to reduce certain birth defects. Proper nutrition and cessation of tobacco, alcohol, and drug use during pregnancy can prevent low birth weight and other complications.
Secondary prevention involves efforts to facilitate early detection and treatment of diseases during pregnancy. Prenatal care provides early and continuous assessment of the pregnant woman, and includes early detection of preeclampsia, syphilis, and tuberculosis.
Tertiary prevention attempts to avert severe complications resulting from diseases during pregnancy. Examples of tertiary prevention include the administration of antibiotics in the treatment of puerperal infection, magnesium to prevent eclampsia (convulsions) in women affected by severe preeclampsia, and transfusion of blood products when obstetrical hemorrhage occurs. Regionalization of perinatal health services, so that high-risk women deliver only in hospitals equipped to deal with potential complications, plays an important role in tertiary prevention.
Much of the improvement in maternal and infant health is attributable to improved health conditions such as better sanitation, sewage control, and safer water supplies. Continued improvement is likely to come from social and behavioral changes rather than from advancement in medical care. Such developments as the expansion in the availability of legal abortions, increased education for women, and better family planning practices have all contributed to improved maternal and infant health. It is important, therefore, for public health professionals to learn how to better address social and behavioral determinants of health. For example, because smoking cigarettes during pregnancy can cause low birth weight and prematurity, it is important to find out how to stop women from smoking during pregnancy.
Because the health of a baby is tied to health of the mother, efforts to improve pregnancy outcomes must begin with women's health. Current efforts fall short by doing too little too late—to expect prenatal care to reverse all the cumulative effects of risk exposures over the course of a woman's life may be expecting too much. Future efforts should promote health not only during pregnancy, but during all of a woman's life.
(SEE ALSO: Abortion; Abortion Laws; Birthrate; Child Health Services; Child Mortality; Contraception; Family Health; Family Planning Behavior; Fecundity and Fertility; Fetal Alcohol Syndrome; Fetal Death; Folic Acid; Infant Mortality Rate; Maternal and Child Health; Newborn Screening; Planned Parenthood; Prenatal Care; Reproduction; Women's Health)
Barker, D. J. P. (1998). Mothers, Babies and Health in Later Life, 2nd edition. Edinburgh: Churchill Livingstone.
Brown, S. S, and Eisenberg, L., eds. (1995). The Best Intentions: Unintended Pregnancy and the Well-Being of Children and Families. Washington, DC: National Academy Press.
Centers for Disease Control and Prevention (2000). "Abortion Surveillance: Preliminary Analysis—United States, 1997." Morbidity and Mortality Weekly Report 48:1171–1174.
Cunningham, F. G.; MacDonald, P. C.; Gant, N. F.; Leveno, K. J.; and Gilstrap, L. C. (1997). Williams Obstetrics, 20th edition. Norwalk, CT: Appleton & Lange.
Curtin, S. C., and Martin, J. A. (2000). "Births: Preliminary Data for 1999." National Vital Statistics Reports 48:14. Hyattsville, MD: National Center for Health Statistics.
Moore, K. L. (1988). Essentials of Human Embryology. Toronto: Decker.
Smedley, B. D., and Syme, S. L., eds. (2000). Promoting Health: Intervention Strategies from Social and Behavioral Research. Washington, DC: National Academy Press.
— MICHAEL C. LU
The folklore of pregnancy, like that of childbirth, is inadequately recorded in England. The process was jokingly compared with baking bread; a pregnant woman is still said to ‘have a bun in the oven’, and a mentally impaired child to be ‘half-baked’. Various signs were thought to indicate the baby's sex. Ancient Greek authorities had taught that a male foetus lay to the right of the mother's womb and affected the right side of her body, and this notion can be found as late as 1724 in Jane Sharp's The Compleat Midwife's Companion: ‘If it be a Boy, she is better Coloured, her Right Breast will swell more, for Males lie most on the right side and her Belly especially on the right side and her Belly especially on that side lieth rounder and more tumefied and the Child will be first felt to move on that side, the Woman is more cheerful and in better Health, her Pains are not so often or so great’ (cited in Chamberlain, 1981: 190).
Women still pass on such tips to one another, though often with amusement rather than belief. Some say one can tell the baby's sex by whether it is carried high or low, and whether it kicks to the right or to the left; others, that ‘boy baby bumps are all out at the front, while girl baby bumps are spread round the side as well’. Many say boy babies kick harder (‘He'll be a footballer!’), but the reverse is recorded too: ‘If you don't feel much movement from the baby it is a boy’ (Chamberlain, 1981: 241).
A divination frequently mentioned is to suspend a wedding ring or a key over the pregnant woman's womb on a thread, or one of her own hairs, to see if it spins clockwise or anti-clockwise, or straight; however, informants disagree on which movement means which sex (Opie and Tatem, 1989: 302-3; Chamberlain, 1981: 241; Sutton, 1992: 57).
Blemishes in a newborn infant were blamed on the circumstances of its conception, or events during the mother's pregnancy. The best known is the harelip, caused by a hare crossing the mother's path, but virtually any troubles could be explained this way. If, for instance, a child had an ugly birthmark, it would be said to resemble something the mother had stared too hard at, or been frightened by, or longed in vain to eat. Examples of this belief can be found from the 16th century to the present. A woman from Hackthorn (Lincolnshire) recalled in the 1980s:
I knew of a child who was born with a perfect mouse on his wrist. His mother had gone into the pantry and had seen a mouse (it's the truth I'm telling you) and she grabbed her wrist like this and the child was born with the shape of a mouse on his wrist. My husband used to say it was balderdash, but it's true. Anyway, he had to go into hospital to have it taken off, so there. (Sutton, 1992: 56)
The story about this unfortunate being is as follows. Shortly before she was born, a woman came begging to her mother's door with two or three little children. Her mother was angry and ordered the woman away. ‘Get away with your young frogs,’ she said. And the child she was expecting was born partly in the form of a frog, as a punishment and a curse upon her. (Kilvert's Diary, ed. W. Plomer (1960),
The period of time (about 280 days) between conception and birth. Although women with a history of poor health may be prescribed rest at various stages of pregnancy, many women exercise and even compete during pregnancy with no ill effects. It is generally agreed that as long as an exercise programme is properly designed, the benefits of exercise during pregnancy outweigh the potential risks. However, all pregnant women should obtain medical clearance before engaging in exercise and should acquire expert advice in designing an individualized programme. The best exercises are non-weight bearing (e.g. cycling and swimming). Exertion levels should be individually determined. Contact sports, exercises in the supine position, and exercising in a warm, humid environment should be avoided. It is important that pregnant women drink plenty of liquids before and after exercise to avoid dehydration. Maximal physical exertion is generally not recommended after the fifth month of pregnancy.
One test that has been used to determine pregnancy uses blood or urine samples to detect a hormone known as BhCG, found exclusively in pregnant women. Later, prenatal diagnostic tests such as alpha fetoprotein, amniocentesis, and chorionic villus sampling may be performed as screening measures for congenital defects. Ultrasound, a sonar device using high-frequency wavelengths, is used to detect defects, measure fetal heartbeat, and monitor growth of a fetus.
Complications of pregnancy include eclampsia, premature birth, and erythroblastosis fetalis (Rh incompatibility). Ectopic pregnancy, in which the fetus begins to develop outside the uterus, often in a fallopian tube, is another complication. It is often the result of scarring from a sexually transmitted disease. Smoking has been linked to low-birth weight infants; alcohol consumption during pregnancy has been linked to a group of defects called fetal alcohol syndrome.
The technology relating to pregnancy has made great advances and has created a number of ethical issues. Many women in their 40s are now able to sustain successful pregnancies, due to technological devices that carefully monitor the progress of the fetus. In vitro fertilization and other infertility treatments have allowed even postmenopausal women to give birth. The use of fertility drugs has led to a marked increase in multiple births. Abortion, in which pregnancy is terminated prior to birth, has long been a subject of heated debate, and surrogate motherhood (see surrogate mother) has also raised ethical issues in recent years.
See J. T. Queenan and C. N. Queenan, ed. A New Life (1992); C. A. Bean, Methods of Childbirth (1990);; Boston Women's Health Book Collective, Our Bodies, Ourselves for the New Century (1998).
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The condition of having a developing embryo or fetus in the body, after union of an ovum and spermatozoon. The duration of pregnancy in each animal species varies widely. See also gestation.
The gestational process, comprising the growth and development within a woman of a new individual from conception through the embryonic and fetal periods to birth. Pregnancy lasts approximately 266 days from the day of fertilization, but is clinically considered to last 280 days (40 weeks, or 10 lunar months) from the first day of the last menstrual period.
|Classification and external resources|
A pregnant woman.
Pregnancy is the fertilization and development of one or more offspring, known as an embryo or fetus, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets. Childbirth usually occurs about 38 weeks after conception; in women who have a menstrual cycle length of four weeks, this is approximately 40 weeks from the start of the last normal menstrual period (LNMP). Human pregnancy is the most studied of all mammalian pregnancies. Conception can be achieved through sexual intercourse or assisted reproductive technology.
An embryo is the developing offspring during the first 8 weeks following conception, and subsequently the term fetus is used until birth. 40% of pregnancies in the United States and United Kingdom are unplanned, and between a quarter and half of those unplanned pregnancies were unwanted pregnancies. Of those unintended pregnancies that occurred in the US, 60% of the women used birth control to some extent during the month pregnancy occurred.
In many societies' medical or legal definitions, human pregnancy is somewhat arbitrarily divided into three trimester periods, as a means to simplify reference to the different stages of prenatal development. The first trimester carries the highest risk of miscarriage (natural death of embryo or fetus). During the second trimester, the development of the fetus can be more easily monitored and diagnosed. The beginning of the third trimester often approximates the point of viability, or the ability of the fetus to survive, with or without medical help, outside of the uterus.
One scientific term for the state of pregnancy is gravidity (adjective "gravid"), Latin for "heavy" and a pregnant female is sometimes referred to as a gravida. Similarly, the term parity (abbreviated as "para") is used for the number of times a female has given birth, counting twins and other multiple births as one pregnancy, and usually including stillbirths. Medically, a woman who has never been pregnant is referred to as a nulligravida, a woman who is (or has been only) pregnant for the first time as a primigravida, and a woman in subsequent pregnancies as a multigravida or multiparous. Hence, during a second pregnancy a woman would be described as gravida 2, para 1 and upon live delivery as gravida 2, para 2. An in-progress pregnancy, as well as abortions, miscarriages, or stillbirths account for parity values being less than the gravida number. In the case of twins, triplets etc., gravida number and parity value are increased by one only. Women who have never carried a pregnancy achieving more than 20 weeks of gestation age are referred to as nulliparous.
Although pregnancy begins with implantation, the process leading to pregnancy occurs earlier as the result of the female gamete, or oocyte, merging with the male gamete, spermatozoon. In medicine, this process is referred to as fertilization; in lay terms, it is more commonly known as "conception." After the point of fertilization, the fused product of the female and male gamete is referred to as a zygote or fertilized egg. The fusion of male and female gametes usually occurs following the act of sexual intercourse, resulting in spontaneous pregnancy. However, the advent of assisted reproductive technology such as artificial insemination and in vitro fertilisation have made achieving pregnancy possible without engaging in sexual intercourse. This approach may be undertaken as a voluntary choice or due to infertility.
The process of fertilization occurs in several steps, and the interruption of any of them can lead to failure. Through fertilization, the egg is activated to begin its developmental process, and the haploid nuclei of the two gametes come together to form the genome of a new diploid organism.
At the beginning of the process, the sperm undergoes a series of changes, as freshly ejaculated sperm is unable or poorly able to fertilize. The sperm must undergo capacitation in the female's reproductive tract over several hours, which increases its motility and destabilizes its membrane, preparing it for the acrosome reaction, the enzymatic penetration of the egg's tough membrane, the zona pellucida, which surrounds the oocyte. The sperm and the egg cell, which has been released from one of the female's two ovaries, unite in one of the two fallopian tubes. The fertilized egg, known as a zygote, then moves toward the uterus, a journey that can take up to a week to complete. Cell division begins approximately 24 to 36 hours after the male and female cells unite. Cell division continues at a rapid rate and the cells then develop into what is known as a blastocyst. The blastocyst is made up of three layers: the ectoderm (which will become the skin and nervous system), the endoderm (which will become the digestive and respiratory systems), and the mesoderm (which will become the muscle and skeletal systems). Finally, the blastocyst arrives at the uterus and attaches to the uterine wall, a process known as implantation.
The mass of cells, now known as an embryo, begins the embryonic stage, which continues until cell differentiation is almost complete at eight weeks. Structures important to the support of the embryo develop, including the placenta and umbilical cord. During this time, cells begin to differentiate into the various body systems. The basic outlines of the organ, body, and nervous systems are established. By the end of the embryonic stage, the beginnings of features such as fingers, eyes, mouth, and ears become visible.
Once cell differentiation is mostly complete, the embryo enters the final stage and becomes known as a fetus. The early body systems and structures that were established in the embryonic stage continue to develop. Sex organs begin to appear during the third month of gestation. The fetus continues to grow in both weight and length, although the majority of the physical growth occurs in the last weeks of pregnancy.
Healthcare professionals name three different dates as the start of pregnancy:
Since these are spread over a significant period of time, the duration of pregnancy necessarily depends on the date selected as the starting point chosen.
As measured on a reference group of women with a menstrual cycle of exactly 28-days prior to pregnancy, and who had spontaneous onset of labor, the mean pregnancy length has been estimated to be 283.4 days of gestational age as timed from the first day of the last menstrual period as recalled by the mother, and 280.6 days when the gestational age was retrospectively estimated by obstetric ultrasound measurement of the fetal biparietal diameter (BPD) in the second trimester. Other algorithms take into account a variety of other variables, such as whether this is the first or subsequent child (i.e., pregnant woman is a primipara or a multipara, respectively), the mother's race, parental age, length of menstrual cycle, and menstrual regularity), but these are rarely used by healthcare professionals. In order to have a standard reference point, the normal pregnancy duration is generally assumed to be 280 days (or 40 weeks) of gestational age.
There is a standard deviation of 8–9 days surrounding due dates calculated with even the most accurate methods. This means that fewer than 5 percent of births occur on the day of being 40 weeks of gestational age; 50 percent of births are within a week of this duration, and about 80 percent are within 2 weeks. It is much more useful and accurate, therefore, to consider a range of due dates, rather than one specific day, with some online due date calculators providing this information.
The most common system used among healthcare professionals is Naegele's rule, which was developed in the early 19th century. This calculates the expected due date from the first day of the last normal menstrual period (LMP or LNMP) regardless of factors known to make this inaccurate, such as a shorter or longer menstrual cycle length. Pregnancy most commonly lasts for 40 weeks according to this LNMP-based method, assuming that the woman has a predictable menstrual cycle length of close to 28 days and conceives on the 14th day of that cycle
Accurate dating of pregnancy is important, because it is used in calculating the results of various prenatal tests, (for example, in the triple test). A decision may be made to induce labour if a fetus is perceived to be overdue. Furthermore, if LMP and ultrasound dating predict different respective due dates, with the latter being later, this might signify slowed fetal growth and therefore require closer review.
The age of fetal viability has been receding because of continued medical progress. Whereas it used to be 28 weeks, it has been brought back to as early as 23, or even 22 weeks in some countries.
Pregnancy is considered "at term" when gestation has lasted 37 complete weeks (occurring at the transition from the 37th to the 38th week of gestation), but is less than 42 weeks of gestational age (occurring at the transition from the 42st week to the 43rd week of gestation, or between 259 and 294 days since LMP). "Full term" refers to the gestation having lasted 40 weeks from the first day of the mother's last menstrual period. This is the end of gestation on average. Alternatively expressed, this corresponds to a gestational age of 40 weeks and 0 days, or 280 days, or approximately 9 months, and occurs at the transition from the 40th to the 41st week of gestation. On average, it corresponds to an embryonic age of 38 weeks or 266 days.
Events before completion of 37 weeks (259 days) are considered preterm; from week 42 (294 days) events are considered postterm. When a pregnancy exceeds 42 weeks (294 days), the risk of complications for both the woman and the fetus increases significantly. Therefore, in an otherwise uncomplicated pregnancy, obstetricians usually prefer to induce labour at some stage between 41 and 42 weeks.
Birth before 39 weeks, even if considered "at term", increases the risk of complications and premature death, from factors including underdeveloped lungs, infection due to underdeveloped immune system, problems feeding due to underdeveloped brain, and jaundice from underdeveloped liver. Some hospitals in the United States have noted a significant increase in neonatal intensive care unit patients when women schedule deliveries for convenience and are taking steps to reduce induction for non-medical reasons. Complications from Caesarean section are more common than for live births.
Recent medical literature prefers the terminology preterm and postterm to premature and postmature. Preterm and postterm are unambiguously defined as above, whereas premature and postmature have historical meaning and relate more to the infant's size and state of development rather than to the stage of pregnancy.
Childbirth is the process whereby an infant is born.
A woman is considered to be in labour when she begins experiencing regular uterine contractions, accompanied by changes of her cervix – primarily effacement and dilation. While childbirth is widely experienced as painful, some women do report painless labours, while others find that concentrating on the birth helps to quicken labour and lessen the sensations. Most births are successful vaginal births, but sometimes complications arise and a woman may undergo a cesarean section.
During the time immediately after birth, both the mother and the baby are hormonally cued to bond, the mother through the release of oxytocin, a hormone also released during breastfeeding. Studies show that skin-to-skin contact between a mother and her newborn immediately after birth is beneficial for both the mother and baby. A review done by the World Health Organization found that skin-to-skin contact between mothers and babies after birth reduces crying, improves mother-infant interaction, and helps mothers to breastfeed successfully. They recommend that neonates be allowed to bond with the mother during their first two hours after birth, the period that they tend to be more alert than in the following hours of early life.
The postnatal period begins immediately after the birth of a child and then extends for about six weeks. During this period, the mother's body begins the return to prepregnancy conditions that includes changes in hormone levels and uterus size.
The beginning of pregnancy may be detected either based on symptoms by the pregnant woman herself, or by using medical tests with or without the assistance of a medical professional. Denial of pregnancy, the refusal to acknowledge that one is pregnant, occurs in approximately 1 in 475 women at 20 weeks and 1 in 2500 women at delivery. Some non pregnant women have a very strong belief that they are pregnant along with some of the physical changes. This condition is known as pseudocyesis or false pregnancy.
Most pregnant women experience a number of symptoms, which can signify pregnancy. The symptoms can include nausea and vomiting, excessive tiredness and fatigue, cravings for certain foods that are not normally sought out, and frequent urination particularly during the night.
A number of early medical signs are associated with pregnancy. These signs typically appear, if at all, within the first few weeks after conception. Although not all of these signs are universally present, nor are all of them diagnostic by themselves, taken together they make a presumptive diagnosis of pregnancy. These signs include the presence of human chorionic gonadotropin (hCG) in the blood and urine, missed menstrual period, implantation bleeding that occurs at implantation of the embryo in the uterus during the third or fourth week after last menstrual period, increased basal body temperature sustained for over 2 weeks after ovulation, Chadwick's sign (darkening of the cervix, vagina, and vulva), Goodell's sign (softening of the vaginal portion of the cervix), Hegar's sign (softening of the uterus isthmus), and pigmentation of linea alba – Linea nigra, (darkening of the skin in a midline of the abdomen, caused by hyperpigmentation resulting from hormonal changes, usually appearing around the middle of pregnancy). Breast tenderness is common during the first trimester, and is more common in women who are pregnant at a young age.
Despite all the signs, some women may not realize they are pregnant until they are far along in pregnancy. In some cases, a few have not been aware of their pregnancy until they begin labour. This can be caused by many factors, including irregular periods (quite common in teenagers), certain medications (not related to conceiving children), and obese women who disregard their weight gain. Others may be in denial of their situation.
Pregnancy detection can be accomplished using one or more various pregnancy tests, which detect hormones generated by the newly formed placenta. Blood and urine tests can detect pregnancy 12 days after implantation. Blood pregnancy tests are more sensitive than urine tests (giving fewer false negatives). Home pregnancy tests are urine tests, and normally detect a pregnancy 12 to 15 days after fertilization. A quantitative blood test can determine approximately the date the embryo was conceived. Testing 48 hours apart can provide useful information regarding how the pregnancy is doing. A single test of progesterone levels can also help determine how likely a fetus will survive in those with a threatened miscarriage (bleeding in early pregnancy).
An early obstetric ultrasonography can determine the age of the pregnancy fairly accurately. In practice, medical professionals typically express the age of a pregnancy (i.e., an "age" for an embryo) in terms of "menstrual date" based on the first day of a woman's last menstrual period, as the woman reports it. Unless a woman's recent sexual activity has been limited, she has been charting her cycles, or the conception is the result of some types of fertility treatment (such as IUI or IVF), the exact date of fertilization is unknown. Without symptoms such as morning sickness, often the only visible sign of a pregnancy is an interruption of the woman's normal monthly menstruation cycle, (i.e., a "late period"). Hence, the "menstrual date" is simply a common educated estimate for the age of a fetus, which is an average of 2 weeks later than the first day of the woman's last menstrual period. The term "conception date" may sometimes be used when that date is more certain, though even medical professionals can be imprecise with their use of the two distinct terms. The due date can be calculated by using Naegele's rule. The expected date of delivery may also be calculated from sonogram measurement of the fetus. This method is slightly more accurate than methods based on LMP. Additional obstetric diagnostic techniques can estimate the health and presence or absence of congenital diseases at an early stage.
One way to observe prenatal development is via ultrasound images. Ultrasound imaging before 24 weeks can help determine the due date and detect multiple pregnancies however in those who are at low risk it is unclear if this makes a significant difference in outcomes. Routine ultrasound imaging after 24 weeks gestation does not improve outcomes in either the mother or the baby and might increase the risk of a cesarean section. It is thus is not recommended. Modern 3D ultrasound images provide greater detail for prenatal diagnosis than the older 2D ultrasound technology. While 3D is popular with parents desiring a prenatal photograph as a keepsake, both 2D and 3D are discouraged by the FDA for non-medical use, but there are no definitive studies linking ultrasound to any adverse medical effects. The following 3D ultrasound images were taken at different stages of pregnancy:
75-mm fetus (about 14 weeks gestational age)
Pregnancy is typically broken into three periods, or trimesters, each of about three months. Obstetricians define each trimester as lasting for 14 weeks, resulting in a total duration of 42 weeks, although the average duration of pregnancy is actually about 40 weeks.While there are no hard and fast rules, these distinctions are useful in describing the changes that take place over time.
Traditionally, medical professionals have measured pregnancy from a number of convenient points, including the day of last menstruation, ovulation, fertilization, implantation and chemical detection. In medicine, pregnancy is often defined as beginning when the developing embryo becomes implanted in the endometrial lining of a woman's uterus. Most pregnant women do not have any specific signs or symptoms of implantation, although it is not uncommon to experience minimal bleeding. After implantation, the uterine endometrium is called the decidua. The placenta, which is formed partly from the decidua and partly from outer layers of the embryo, connects the developing embryo to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. The umbilical cord is the connecting cord from the embryo or fetus to the placenta. The developing embryo undergoes tremendous growth and changes during the process of fetal development.
Morning sickness occurs in about seventy percent of all pregnant women, and typically improves after the first trimester. Although described as "morning sickness", women can experience this nausea during afternoon, evening, and throughout the entire day.
Shortly after conception, the nipples and areolas begin to darken due to a temporary increase in hormones. This process continues throughout the pregnancy.
The first 12 weeks of pregnancy are considered to make up the first trimester. The first two weeks from the first trimester are calculated as the first two weeks of pregnancy even though the pregnancy does not actually exist. These two weeks are the two weeks before conception and include the woman's last period.
The third week is the week in which fertilization occurs and the 4th week is the period when implantation takes place. In the 4th week, the fecundated egg reaches the uterus and burrows into its wall which provides it with the nutrients it needs. At this point, the zygote becomes a blastocyst and the placenta starts to form. Moreover, most of the pregnancy tests may detect a pregnancy beginning with this week.
The 5th week marks the start of the embryonic period. This is when the embryo's brain, spinal cord, heart and other organs begin to form. At this point the embryo is made up of three layers, of which the top one (called the ectoderm) will give rise to the embryo's outermost layer of skin, central and peripheral nervous systems, eyes, inner ear, and many connective tissues. The heart and the beginning of the circulatory system as well as the bones, muscles and kidneys are made up from the mesoderm (the middle layer). The inner layer of the embryo will serve as the starting point for the development of the lungs, intestine and bladder. This layer is referred to as the endoderm. An embryo at 5 weeks is normally between 1⁄16 and 1⁄8 inch (1.6 and 3.2 mm) in length.
In the 6th week, the embryo will be developing basic facial features and its arms and legs start to grow. At this point, the embryo is usually no longer than 1⁄6 to 1⁄4 inch (4.2 to 6.4 mm). In the following week, the brain, face and arms and legs quickly develop. In the 8th week, the embryo starts moving and in the next 3 weeks, the embryo's toes, neck and genitals develop as well. According to the American Pregnancy Association, by the end of the first trimester, the fetus will be about 3 inches (76 mm) long and will weigh approximately 1 ounce (28 g). Once pregnancy moves into the second trimester, all the risks of miscarriage and birth defects occurring drop drastically. Progesterone has noticeable effects on respiratory physiology, increasing minute ventilation by 40% in the first trimester.
Weeks 13 to 28 of the pregnancy are called the second trimester. Most women feel more energized in this period, and begin to put on weight as the symptoms of morning sickness subside and eventually fade away. The uterus, the muscular organ that holds the developing fetus, can expand up to 20 times its normal size during pregnancy.
Although the fetus begins to move and takes a recognizable human shape during the first trimester, it is not until the second trimester that movement of the fetus, often referred to as "quickening", can be felt. This typically happens in the fourth month, more specifically in the 20th to 21st week, or by the 19th week if the woman has been pregnant before. However, it is not uncommon for some women not to feel the fetus move until much later. The placenta fully functions at this time and the fetus makes insulin and urinates. The reproductive organs distinguish the fetus as male or female. During the second trimester, most women begin to wear maternity clothes.
Final weight gain takes place, which is the most weight gain throughout the pregnancy. The fetus will be growing the most rapidly during this stage, gaining up to 28 g per day. The woman's belly will transform in shape as the belly drops due to the fetus turning in a downward position ready for birth. During the second trimester, the woman's belly would have been very upright, whereas in the third trimester it will drop down quite low, and the woman will be able to lift her belly up and down. The fetus begins to move regularly, and is felt by the woman. Fetal movement can become quite strong and be disruptive to the woman. The woman's navel will sometimes become convex, "popping" out, due to her expanding abdomen. This period of her pregnancy can be uncomfortable, causing symptoms like weak bladder control and backache. Movement of the fetus becomes stronger and more frequent and via improved brain, eye, and muscle function the fetus is prepared for ex utero viability. The woman can feel the fetus "rolling" and it may cause pain or discomfort when it is near the woman's ribs and spine.
There is head engagement in the third trimester, that is, the fetal head descends into the pelvic cavity so that only a small part (or none) of it can be felt abdominally. The perenium and cervix are further flattened and the head may be felt vaginally. Head engagement is known colloquially as the baby drop, and in natural medicine as the lightening because of the release of pressure on the upper abdomen and renewed ease in breathing. However, it severely reduces bladder capacity, increases pressure on the pelvic floor and the rectum, and the mother may experience the perpetual sensation that the fetus will "fall out" at any moment. It is also during the third trimester that maternal activity and sleep positions may affect fetal development due to restricted blood flow. For instance, the enlarged uterus may impede blood flow by compressing the lower pressured vena cava, with the left lateral laying positions appearing to providing better oxygenation to the infant.
It is during this time that a baby born prematurely may survive. The use of modern medical intensive care technology has greatly increased the probability of premature babies surviving, and has pushed back the boundary of viability to much earlier dates than would be possible without assistance. In spite of these developments, premature birth remains a major threat to the fetus, and may result in ill health in later life, even if the baby survives.
Prenatal development is divided into two primary biological stages. The first is the embryonic stage, which lasts for about two months. At this point, the fetal stage begins. At the beginning of the fetal stage, the risk of miscarriage decreases sharply, and all major structures including the head, brain, hands, feet, and other organs are present, and they continue to grow and develop. When the fetal stage commences, a fetus is typically about 30 mm (1.2 inches) in length, and the heart can be seen beating via ultrasound; the fetus can be seen making various involuntary motions at this stage.
Electrical brain activity is first detected between the 5th and 6th week of gestation, though this is still considered primitive neural activity rather than the beginning of conscious thought, something that develops much later in fetation. Synapses begin forming at 17 weeks, and at about week 28 begin to multiply at a rapid pace which continues until 3 to 4 months after birth.
During pregnancy, the woman undergoes many physiological changes, which are entirely normal, including cardiovascular, hematologic, metabolic, renal and respiratory changes that become very important in the event of complications. The body must change its physiological and homeostatic mechanisms in pregnancy to ensure the fetus is provided for. Increases in blood sugar, breathing and cardiac output are all required. Levels of progesterone and oestrogens rise continually throughout pregnancy, suppressing the hypothalamic axis and subsequently the menstrual cycle.
Many women and medical professionals mistakenly think that breastfeeding causes their breasts to sag (medically referred to as ptosis), and as a result some are reluctant to nurse their infants. In February 2009, Cheryl Cole told British Vogue that she hesitated to breastfeed because of the effect it might have on her breasts. "I want to breastfeed," she said, "but I've seen what it can do, so I may have to reconsider." Research shows that breastfeeding is less of a factor than previously thought. The main risk factors for ptosis are cigarette smoking, a woman's body mass index (BMI), her number of pregnancies, her breast cup size before pregnancy, and age.
Prenatal medical care is the medical and nursing care recommended for women before and during pregnancy. The aim of good prenatal care is to identify any potential problems early, to prevent them if possible (through recommendations on adequate nutrition, exercise, vitamin intake etc.), and to manage problems, possibly by directing the woman to appropriate specialists, hospitals, etc. if necessary.
A balanced, nutritious diet is an important aspect of a healthy pregnancy. Eating a healthy diet, balancing carbohydrates, fat, and proteins, and eating a variety of fruits and vegetables, usually ensures good nutrition. Those whose diets are affected by health issues, religious requirements, or ethical beliefs may choose to consult a health professional for specific advice.
Adequate periconceptional folic acid (also called folate or Vitamin B9) intake has been shown to decrease the risk of fetal neural tube defects such as spina bifida, a serious birth defect. The neural tube develops during the first 28 days of pregnancy, explaining the necessity to guarantee adequate periconceptional folate intake. Folate (from folia, leaf) is abundant in spinach (fresh, frozen, or canned), and is found in green leafy vegetables e.g. salads, beets, broccoli, asparagus, citrus fruits and melons, chickpeas (i.e. in the form of hummus or falafel), and eggs. In the United States and Canada, most wheat products (flour, noodles) are fortified with folic acid.
DHA omega-3 is a major structural fatty acid in the brain and retina, and is naturally found in breast milk. It is important for the woman to consume adequate amounts of DHA during pregnancy and while nursing to support her well-being and the health of her infant. Developing infants cannot produce DHA efficiently, and must receive this vital nutrient from the woman through the placenta during pregnancy and in breast milk after birth.
Several micronutrients are important for the health of the developing fetus, especially in areas of the world where insufficient nutrition is prevalent. In developed areas, such as Western Europe and the United States, certain nutrients such as Vitamin D and calcium, required for bone development, may require supplementation.
Dangerous bacteria or parasites may contaminate foods, including Listeria and Toxoplasma gondii. Careful washing of fruits and raw vegetables may remove these pathogens, as may thoroughly cooking leftovers, meat, or processed meat. Soft cheeses may contain Listeria; if milk is raw, the risk may increase. Cat feces pose a particular risk of toxoplasmosis. Pregnant women are also more prone to Salmonella infections from eggs and poultry, which should be thoroughly cooked. Practicing good hygiene in the kitchen can reduce these risks.
Caloric intake should be increased to ensure proper development of the fetus. The amount of weight gained during a single pregnancy varies. The Institute of Medicine recommends an overall pregnancy weight gain for those of normal weight (body mass index of 18.5–24.9), of 11.3–15.9 kg (25–35 pounds). Women who are underweight (BMI of less than 18.5), should gain between 12.7–18 kg (28–40 lbs), while those who are overweight (BMI of 25–29.9) are advised to gain between 6.8–11.3 kg (15–25 lbs) and those who are obese (BMI>30) should gain between 5–9 kg (11–20 lbs).
During pregnancy, insufficient or excessive weight gain can compromise the health of the mother and fetus. Women are encouraged to choose a healthy diet regardless of pre-pregnancy weight. Exercise during pregnancy, such as walking and swimming, is recommended for healthy pregnancies. Exercise has notable health benefits for both mother and baby, including preventing excessive weight gain.
Drugs used during pregnancy can have temporary or permanent effects on the fetus. Therefore many physicians would prefer not to prescribe for pregnant women, the major concern being over teratogenicity of the drugs.
Drugs have been classified into categories A,B,C,D and X based on the Food and Drug Administration (FDA) rating system to provide therapeutic guidance based on potential benefits and fetal risks. Drugs, including some multivitamins, that have demonstrated no fetal risks after controlled studies in humans are classified as Category A. On the other hand drugs like thalidomide with proven fetal risks that outweigh all benefits are classified as Category X.
Various toxins pose a significant hazard to fetuses during development. A 2011 study found that virtually all U.S. pregnant women carry multiple chemicals, including some banned since the 1970s, in their bodies. Researchers detected polychlorinated biphenyls, organochlorine pesticides, perfluorinated compounds, phenols, polybrominated diphenyl ethers, phthalates, polycyclic aromatic hydrocarbons, perchlorate PBDEs, compounds used as flame retardants, and dichlorodiphenyltrichloroethane (DDT), a pesticide banned in the United States in 1972, in the bodies of 99 to 100 percent of the pregnant women they tested. Bisphenol A (BPA) was identified in 96 percent of the women surveyed. Several of the chemicals were at the same concentrations that have been associated with negative effects in children from other studies and it is thought that exposure to multiple chemicals can have a greater impact than exposure to only one substance.[medical citation needed]
Most women can continue to engage in sexual activity throughout pregnancy. Most research suggests that during pregnancy both sexual desire and frequency of sexual relations decrease. In context of this overall decrease in desire, some studies indicate a second-trimester increase, preceding a decrease during the third trimester. Some individuals are sexually attracted to pregnant women (pregnancy fetishism, also known as maiesiophilia).
Sex during pregnancy is a low-risk behavior except when the healthcare provider advises that sexual intercourse be avoided for particular medical reasons. Otherwise, for a healthy pregnant woman who is not ill or weak, there is no safe or right way to have sex during pregnancy: it is enough to apply the common sense rule that both partners avoid putting pressure on the uterus, or a partner's full weight on a pregnant belly.
Regular aerobic exercise during pregnancy appears to improve (or maintain) physical fitness; however, the quality of the research is poor and the data was insufficient to infer important risks or benefits for the mother or infant.
The Clinical Practice Obstetrics Committee of Canada recommends that "All women without contraindications should be encouraged to participate in aerobic and strength-conditioning exercises as part of a healthy lifestyle during their pregnancy". Although an upper level of safe exercise intensity has not been established, women who were regular exercisers before pregnancy and who have uncomplicated, healthy pregnancies should be able to engage in high intensity exercise programs, such as jogging and aerobics for less than 45 minutes, with no adverse effects if they are mindful of the possibility that they may need to increase their energy intake and are careful to not become overheated. In the absence of either medical or obstetric complications, they advise an accumulation of 30 minutes a day of exercise on most if not all days of the week. In general, participation in a wide range of recreational activities appears to be safe, with the avoidance of those with a high risk of falling such as horseback riding or sking or those that carry a risk of abdominal trauma, such as soccer or hockey.
The American College of Obstetricians and Gynecologists reports that in the past, the main concerns of exercise in pregnancy were focused on the fetus and any potential maternal benefit was thought to be offset by potential risks to the fetus. However, they write that more recent information suggests that in the uncomplicated pregnancy, fetal injuries are highly unlikely. They do, however, list several circumstances when a woman should contact her health care provider before continuing with an exercise program. Contraindications include: Vaginal bleeding, dyspnea before exertion, dizziness, headache, chest pain, muscle weakness, preterm labor, decreased fetal movement, amniotic fluid leakage, and calf pain or swelling (to rule out thrombophlebitis). 
Each year, according to the WHO, ill-health as a result of pregnancy is experienced (sometimes permanently) by more than 20 million women around the world. Furthermore, the "lives of eight million women are threatened, and more than 500,000 women are estimated to have died in 1995 as a result of causes related to pregnancy and childbirth."
Pregnancy poses varying levels of health risk for women, depending on their medical profile before pregnancy.
The following are some of the complaints that may occur during and/or after pregnancy due to the many changes which pregnancy causes in a woman's body:
An ectopic pregnancy is a complication of pregnancy in which the embryo implants outside the uterine cavity. With rare exceptions, ectopic pregnancies are not viable. Most ectopic pregnancies occur in the Fallopian tube (so-called tubal pregnancies), but implantation can also occur in the cervix, ovaries, and abdomen. It should be considered in any woman with abdominal pain or vaginal bleeding who has a positive pregnancy test. An ultrasound showing a gestational sac with fetal heart in a location other than the uterine cavity is clear evidence of an ectopic pregnancy. Tubal ectopic pregnancy is the most common cause of maternal death in the first trimester of pregnancy.
About 1% of pregnancies are in an ectopic location with implantation not occurring inside of the womb, and of these 98% occur in the Fallopian tubes. In a typical ectopic pregnancy, the embryo adheres to the lining of the fallopian tube and burrows into the tubal lining. Most commonly this invades blood vessels which causes bleeding resulting in the expulsion of the implantation from the tube. Termed "tubal abortions", about half of ectopic pregnancies will resolve without treatment. The use of methotrexate treatment for ectopic pregnancy has reduced the need for surgery, but surgical intervention is still required in cases where the Fallopian tube has ruptured or is in danger of doing so. The surgical intervention may be laparoscopic or through a larger incision, known as a laparotomy.
A woman who has had a previous ectopic pregnancy is more likely to have another. The majority of women with ectopic pregnancies have had pelvic inflammatory disease or salpingitis, an inflammation of the fallopian tube. A history of gonorrhea or chlamydia can also cause tubal problems that increase the risk. Endometriosis, a condition that causes the tissue that normally lines the uterus to develop outside the uterus may slightly increase the incidence of an ectopic. The risk is increased in women who have unusually shaped fallopian tubes or tubes which has been damaged, possibly during surgery. Taking medication to stimulate ovulation increases the risk of ectopic pregnancy. Although pregnancy is rare when using birth control pills or an intrauterine device (IUD), if it does occur, it's more likely to be ectopic. Although pregnancy is rare after tubal ligation, if it does occur, it's more likely to be ectopic. A recent meta-analysis of clinical outcomes has shown that cigarette smoking significantly increases the risk of tubal ectopic pregnancy.
In addition to complications of pregnancy that can arise, a woman may have other diseases or conditions (not directly caused by the pregnancy) that may become worse or be a potential risk to the pregnancy.
In Europe, the average childbearing age has been rising continuously for some time. In Western, Northern, and Southern Europe, first-time mothers are on average 26 to 29 years old, up from 23 to 25 years at the start of the 1970s. In a number of European countries (Spain), the mean age of women at first childbirth has now even crossed the 30-year threshold.
This process is not restricted to Europe. Asia, Japan and the United States are all seeing average age at first birth on the rise, and increasingly the process is spreading to countries in the developing world like China, Turkey and Iran. In the U.S., the age of first childbirth was 25 in 2006.
In most cultures, pregnant women have a special status in society and receive particularly gentle care. At the same time, they are subject to expectations that may exert great psychological pressure, such as having to produce a son and heir. In many traditional societies, pregnancy must be preceded by marriage, on pain of ostracism of mother and (illegitimate) child.
Depictions of pregnant women can serve as mystically connotated symbols of fertility. The so-called Venus of Willendorf with its exaggerated female sexual characteristics (huge breasts and belly, prominent mons pubis) has been interpreted as indicative of a fertility cult in paleolithic Europe.
Overall, pregnancy is accompanied by numerous customs that are often subject to ethnological research, often rooted in traditional medicine or religion. The baby shower is an example of a modern custom.
Pregnancy is an important topic in sociology of the family. The prospective child is preliminarily placed into numerous social roles such as prospective heir or welfare recipient. (This may accelerate weddings.) The parents' relationship and the relation between parents and their surroundings are also affected.
An abortion is the termination of an embryo or fetus, either naturally or via medical methods. When done electively, it is more often done within the first trimester than the second, and rarely in the third. Not using contraception, contraceptive failure, poor family planning or rape can lead to undesired pregnancies. Legality of socially indicated abortions varies widely both internationally and through time. In most countries of Western Europe, abortions during the first trimester were a criminal offense a few decades ago but have since been legalized, sometimes subject to mandatory consultations. In Germany, for example, as of 2009 less than 3% of abortions had a medical indication.
Many countries have various legal regulations in place to protect pregnant women and their children. Maternity Protection Convention ensures that pregnant women are exempt from activities such as night shifts or carrying heavy stocks. Maternity leave typically provides paid leave from work during roughly the last trimester of pregnancy and for some time after birth. Notable extreme cases include Norway (8 months with full pay) and the United States (no paid leave at all except in some states). Moreover, many countries have laws against pregnancy discrimination.
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Common misspelling(s) of pregnancies