lactation

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The function of the mammary gland providing milk nourishment to the newborn mammal. This process is under the control of the endocrine and nervous systems. It involves transformation of an inactive duct system to a lobuloalveolar glandular structure during pregnancy, cellular production of the components of milk (galactopoiesis), secretion into the ducts, and ejection under the stimulus of milking or suckling.

Lactation makes demands on the maternal regulation of calcium metabolism. Resorption of bone increases in lactating rats and women, and there is a marked increase in the absorption of calcium from the intestine. The elevated need for calcium results in an increased role for parathyroid hormone, calcitonin, and vitamin D in the regulation of the absorption and utilization of calcium. In humans a concomitant phenomenon frequently associated with lactation is amenorrhea. Consequently in some societies prolonged nursing is used as a birth control technique. See also Mammary gland; Milk.

The process of synthesizing and secreting milk from the breasts.

Lactating women have slightly increased energy and protein requirements compared with those who are not breast-feeding (although considerable reserves of fat are laid down in pregnancy to cope with the stress of lactation), and high requirements for iron and calcium. These increased needs are reflected in the increased reference intakes for lactating women.

Definition

Lactation refers to the formation of milk in the breasts during the period following childbirth. Breastfeeding is the process of the infant obtaining milk by suckling at the breast.

Description

Although breast development begins around puberty, development of mammary function is only completed in pregnancy. During the first half of pregnancy the mammary ducts proliferate and group together to form large lobules. During the second half of pregnancy, secretory activity increases and the alveoli become distended by accumulating colostrum. After 16 weeks of pregnancy, lactation occurs even if the pregnancy does not progress.

The ability of the mammary gland to secrete milk during later pregnancy is called lactogenesis, stage 1. During this time, breast size increases and fat droplets accumulate in the secretory cells. The onset of copious milk secretions after birth is lactogenesis, stage 2, and usually occurs from day two or three to eight days postpartum. During this time, the milk goes through a maturation process to match the infant's needs. Without the hormone prolactin, lactation would not occur. During pregnancy prolactin helps to increase breast mass but does not cause lactation because it is inhibited by the hormone progesterone, which is made by the placenta. The inhibiting influence of progesterone is so strong that lactation is delayed if any of the placenta is retained after birth. Prolactin levels rise and fall in direct proportion to the frequency, intensity, and duration of nipple stimulation from the infant's suckling. During the first week after birth, prolactin levels in breastfeeding women fall about 50 percent. If a mother does not breastfeed, prolactin levels usually reach the levels of the nonpregnant state by seven days postpartum. After milk "comes in" or rapidly increases in volume, lactation is no longer driven by the hormone prolactin. It shifts control to a milk removal driven process, i.e., sucking stimulus. Thus, the initiation of lactation is not driven by breastfeeding, but breastfeeding is necessary for the continuation of lactation.

The breast is not a passive container of milk. It is an organ that actively produces milk due to the stimulus of the infant's sucking; the removal of milk from the breasts causes continued milk production. It is a supply and demand response that regulates the production of milk to match the intake of the infant. The composition of breast milk changes to meet the specific needs of the growing infant. In response to suckling, the hormone oxytocin causes the milk ejection reflex or "let-down" reflex to occur. Milk ejection is the forceful expulsion of milk from the alveoli openings. Oxytocin secretion is also nature's way of causing a woman's uterus to contract after birth to control postpartum bleeding and assist in uterine involution. These contractions can continue for up to 20 minutes after feeding and may be painful during the first few days. The benefit of this, however, is that uterine discharge diminishes faster and the uterine involution occurs more quickly.

Colostrum is thick and creamy yellow as compared with mature milk, which is thin and bluish-white. Compared with mature milk, colostrum is richer in protein and minerals and lower in carbohydrates, fat, and some vitamins. The high concentration of total protein and minerals in colostrum gradually changes to meet the infant's needs over the first two to three weeks until lactation is established. The key component in colostrum and breast milk is immunoglobulins or antibodies that serve to protect the infant against infections or viruses. Breast milk also facilitates the development of the infant's own immune system to mature faster. As a result, breast-fed babies have fewer ear infections, diarrhea, rashes, allergies, and other medical problems than bottle-fed babies. Human milk is rich in proteins, lipids, carbohydrates, vitamins, minerals, hormones, enzymes, growth factors, and many types of protective agents. It contains about 10 percent solids for energy and growth and the rest is water, which is essential to maintain hydration. This is also why a breastfed baby does not need additional water. Infants can digest breast milk much more rapidly than formula and, therefore, do not get constipated. On average, it takes about 30 minutes longer to digest formula as opposed to breast milk. Breastfed babies have better cheekbone development and better jaw alignment.

Besides the benefits of the contracting uterus, the process of producing milk burns calories, which helps the mother to lose excess weight gained during pregnancy. After all, that is why pregnant women put on extra fat during pregnancy—energy storage for milk production. Breastfeeding is also related to a lower risk of breast cancer and ovarian cancer. For every year of life spent breastfeeding, a woman's risk of developing breast cancer drops by 4.3 percent and this is on top of the 7 percent reduction she enjoys for every baby to whom she gives birth.

Additionally, there is the convenience. Breast milk is always with the mother. Mothers do not have to store it. It is always at the right temperature. It is free. It does not require sterilization. In fact, it prevents diseases and has protective factors resulting in healthier babies and decreased healthcare costs. It saves money as there is no need to buy formula, bottles, and nipples.

Procedure

It is best to begin breastfeeding immediately after birth as it is an infant's natural instinct to nurse then. Regardless of the baby's initial suckling behavior, this interaction stimulates uterine contractions, promotes colonization of harmless bacteria on the nipple, and helps to protect the infant from pathogenic bacteria. It is an important time to nuzzle. Women breastfeed for a longer duration if feedings are started early. The first several feedings have an imprinting effect. It is recommended to continue feeding about every two to three hours. It is important to remember that all babies are different; some need to nurse almost constantly at first, while others can go much longer between feedings. There are babies and mothers who have no trouble breastfeeding, while others may need some assistance. Once the baby begins to suck, the mother makes sure that the entire dark area around the nipple (areola) is in the baby's mouth. This helps stimulate milk flow and allows the baby to get enough milk. Nipple soreness can be a result of the infant not getting a good grasp of the entire areola. A newborn needs to be fed at least eight to 12 times in 24 hours. Since breast milk is so easily digested, a baby may be hungry again as soon as one and one-half hours after the last feeding.

Mothers need to be comfortable when nursing; therefore, loose, front-opening clothes and a good nursing bra are essential. They need to explore different positions for breastfeeding to determine what is best for them. The cradle hold works well in bed or sitting in a comfortable chair. The football hold is excellent if the woman had a cesarean section. The mother can use pillows to support the baby and a footstool to flatten her

lap. The mother can position the baby's head by snuggling it in one arm and supporting her breast with the other hand by keeping her thumb well above the areola and the rest of the fingers below and under the breast (sometimes called the C-hold). In this position, the mother can lift her breast and guide her nipple in any direction as she helps the baby to take in more of the areola.

For early feedings, the infant should be offered both breasts at each feeding as this stimulates the need-supply response. The length of the feeding is up to the mother. The general rule is to watch the baby, not the clock. If, however, it is a first time mother, 20 to 30 minutes on the first side can be suggested. If the baby falls asleep at the breast, the next feeding should begin with the breast that was not nursed. Mothers can tell if the baby is getting enough milk by checking diapers; a baby who is wetting between four to six disposable diapers (six to eight cloth) and who has three or four bowel movements in 24 hours is getting enough milk.

Common Problems

New mothers may experience nursing problems, including the following:

• Engorgement: Breasts that are too full can prevent the baby from suckling because they cannot be grasped. Expressing milk manually or with a breast pump can alleviate this problem.
• Sore nipples: Transient soreness can occur during the first week postpartum and is usually temporary. Air drying the nipples and rubbing colostrum or breast milk into them provides relief. Prolonged, abnormal soreness lasts longer than a week postpartum. Discontinuing use of soap on breasts while bathing and applying purified lanolin to nipples and air drying them helps.
• Infection: Soreness and inflammation on the breast surface or a fever in the mother may be an indication of breast infection (mastitis). If it is just starting, the mother should drink lots of water and nurse frequently on the affected breast. Antibiotics may be necessary if the infection persists.

Lactation consultants work at almost every hospital where babies are delivered. First-time mothers can request the lactation consultant to visit her. The mother should make a note of the lactation consultant's phone number should problems be encountered after mother and infant go home.

There are no rules about when to stop breastfeeding. A baby needs breast milk for at least the first year of life and it is preferred that no solid food be given for at least the first six months to prevent allergies. As long as a baby eats age-appropriate solid food, the mother may nurse for several years.

Parental Concerns

The majority of illnesses are not transmitted via breast milk; in fact, breast milk prevents many illnesses. However, some viruses, including HIV (the virus that causes AIDS can be passed in breast milk; for this reason, women who are HIV-positive should not breastfeed unless they are living in a country that does not have clean water to make formula. A lack of clean water to make formula could result in an infant dying from diarrhea.

Many medications have not been tested in nursing women, so it is not certain what drugs can affect a breastfed child. A nursing woman should always check with her doctor or lactation consultant before taking any medications, including over-the-counter drugs. The mother can usually take antibiotics without discontinuing breastfeeding.

The following drugs are not safe for a mother to take while she is nursing:

• radioactive drugs for some diagnostic tests
• chemotherapy drugs for cancer
• bromocriptine
• ergotamine
• lithium
• methotrexate
• street drugs (including marijuana, heroin, amphetamines)
• tobacco

Resources

Books

Behrmann, Barbara L. The Breastfeeding Café: Mothers Share the Joys, Challenges, and Secrets of Nursing. Ann Arbor, MI: University of Michigan Press, 2005.

Hanson, Lars A. Immunobiology of Human Milk: How Breastfeeding Protects Babies. Armillo, TX: Pharmasoft Publishing, 2004.

La Leche League International Staff. The Womanly Art of Breastfeeding. East Rutherford, NJ: Penguin Group, 2004.

Lim, Robin. Eating for Two: Recipes for Pregnant and Breastfeeding Women. Berkeley, CA: Celestial Arts Publishing, 2004.

Olds, Sally, et al. Maternal-Newborn Nursing & Women's Health Care, 7th ed. Saddle River, NJ: Prentice Hall, 2004.

Riordan, Jan. Breastfeeding and Human Lactation, 3rd ed. Boston, MA: Jones and Bartlett Publishers, 2004.

Organizations

International Lactation Consultants Association. 1500 Sunday Drive, Suite 102; Raleigh, NC 27607. Web site: www.ilca.org/.

La Leche League International. 1400 North Meacham Rd., Schaumburg, IL 60173. Web site: www.lalecheleague.org/.

National Alliance for Breastfeeding Advocacy. 9684 Oak Hill Drive; Ellicott City, MD 21042. Web site: www.healthfinder.gov/orgs/HR2952.htm.

[Article by: Linda K. Bennington]

Lactation refers to the ability of mammals, warm-blooded, backboned animals, to nourish their young with milk produced by the mammary glands. Many other distinguishing features separate mammals into families that include over four thousand species. The milk of each species is specifically engineered for the growth and developmental needs of that species. In fact studies of the characteristics of the milk can predict the growth rate of both body and brain and the developmental maturity of the offspring at birth. It is also possible to predict the feeding patterns that vary from the whale, which feeds its young every three to four days and has extremely high-fat milk (50 percent), to the human, who initially feeds the infant every two to three hours and has low-protein, low-fat milk (3.4 percent). Some of the world's finest scientists have turned their attention to human lactation and have not only deciphered the micronutrients of human milk but have studied the nutrient needs of the human infant, especially as they pertain to brain development and physical growth. Research also has explained the physiology of human lactation.

Historically, in times of wealth and prosperity, women of higher socioeconomic levels have sought substitute feedings for their infants to "free" themselves of the burden of breast-feeding. Dogma and ritual have developed in different cultures of the world around nursing. It is significant that the Qur'an states that women should nurse their infants for two years. In the Old Testament, the Book of Psalms refers to the value of mother's milk. Pope John Paul II stated that the women of the world should provide their milk for their infants.

When bottle-feeding became more available for the average mother due to the discovery of sterilization, followed by the availability of prepared formulas, the trend toward bottle-feeding increased from 1930 to 1950. Well-educated women led the march to the bottle because they wanted to raise their infants by the book, with scientific information. In the second half of the twentieth century, these same educated women sought a different mode of childbirth in which the mother was prepared and in control. Concomitant with this, well-educated women began looking at breast-feeding as the most appropriate course for their infants.

In 1978 a bipartisan congressional committee charged with the responsibility of designing a health plan for the United States established the year 1990 as the target date for accomplishing several health goals. In addition to statements regarding decreasing hypertension, obesity, and smoking, the committee stated that 75 percent of women should leave the hospital breast-feeding and at least 35 percent should still be breast-feeding at six months. Many of the goals were not accomplished, and in 1990 they were rewritten with a target date of 2000. In 2000 they were rewritten for 2010, aiming at 75 percent of mothers initiating breast-feeding, 50 percent continuing for six months, and 25 percent continuing for a full year. The World Health Organization Code for Infant Feeding was developed in 1981, and the most industrialized countries of the world endorsed this policy, which supported breast-feeding and rejected the promotion of artificial feedings and advertisement of these feedings to the public. The United States did not sign until 1994. The Institute of Medicine, through the Subcommittee on Nutrition during Lactation, confirmed the position that all women, under ordinary circumstances, should breast-feed their infants and further stated that breast-feeding was ideal, even if the mother's diet was not perfect. The American Academy of Pediatrics, joined by the American College of Obstetrics and Gynecology, stated in 1997 that infants should be exclusively breast-fed for five to six months. They further stated that breast-feeding should continue as weaning foods are added through the first year of life and then for as long thereafter as the mother and the infant choose

Significance of Breast-Feeding to Health

Why have all of these important groups spoken out so strongly in favor of breast-feeding? The knowledge that human milk is for the human infant has been accepted for centuries. In the late twentieth century, however, considerable scientific investigation established unequivocally that breast-feeding is associated with a reduced incidence of infection in the infant, including reduced incidences of gastrointestinal, upper and lower respiratory, ear, and urinary tract infections. Immunologic data have shown reduced incidences of childhood-onset cancers, especially lymphoma and acute lymphocytic leukemia. Crohn's disease, celiac disease, and childhood-onset diabetes also are reduced when infants are breast-fed for at least four months. Probably the most dramatic information published in multiple articles is the relationship between breast-feeding and infant development. A study by Niles Newton compared the developmental progress of breast-fed and bottle-fed three-year-olds. Alan Lucas, Ruth Morley, T. J. Cole, and others reported a multisite study that compared premature babies given their mother's milk by feeding tube with infants given premature-infant formula. The group studied them at eighteen months and followed them until seven and a half to eight years of age. The study showed an 8.5-point difference when the data were adjusted for socioeconomic status and education of the mother. The eighteen-year study by L. John Horwood and David M. Fergusson in New Zealand showed a measurable difference at eighteen years of age in school outcomes and behaviors related to whether or not the children were breast-fed in infancy. Although these studies have been criticized for design flaws, many scientists accept their findings. These results are in addition to the compelling psychologic benefit to the mother and the infant in their relationship during breast-feeding.

Facilitating the Decision to Breast-Feed

A mother needs an opportunity to make an informed decision about how to feed her infant. If a mother comes to pregnancy without any information on this process, it is the health care provider's responsibility to see that she is well informed about the benefits of breast-feeding for her baby, for herself, and for society so she can make a decision that will be optimal.

The economic benefit of breast-feeding. A simple calculation of the cost of buying formula does not reflect completely the monetary benefits of breast-feeding. It costs between $60 and $80 a month to purchase infant formula, $700 to $1,000 for the first year of life. Careful studies in controlled populations, such as in health maintenance organizations, have demonstrated in multiple reports that infants who are not breast-fed have an increased number of illnesses, visits to the doctor, prescription medications, and hospitalizations compared with their breast-fed counterparts. The estimate per infant of the health care costs not to breast-feed is between $600 and $1,000 per year. This estimate does not include the reduction in the onset of chronic illnesses that may last a lifetime, such as diabetes, Crohn's disease, and allergies.

Benefits to the mother. The benefits of breast-feeding to the mother are often ignored. Women who breast-feed return to their prepregnant, physiologic states more rapidly. The uterus involutes, the postpartum blood loss is reduced, and the woman returns to her physiologic weight as well. Among other possible benefits are reduced incidences of long-term obesity, breast cancer, ovarian cancer, and most remarkably long-term osteoporosis. Although breast-feeding is not a contraceptive, it significantly affects the fertility in the childbearing years by suppressing ovulation.

Establishing lactation. Critical information about the mother's potential for a good milk supply is obtained during pregnancy. When the obstetrician does the early examination of the breasts in the first trimester, the breasts should be evaluated with respect to their potential for producing milk. Unusually small, unusually large, asym-metric, or tubular-shaped breasts may pose a problem. Prior surgery of the breast should be discussed. Lumpectomies and augmentation mammoplasty are not contraindications. Reduction mammoplasty, however, may pose a problem if the integrity of the ducts was interrupted. The obstetrician should also evaluate the breasts' responses to the hormones of pregnancy, the degree of increase in size of the breasts, and changes in the areola and nipple. The obstetrician should discuss with the mother her intentions to breast-feed and address any questions she may have. The mother should be encouraged to attend breast-feeding preparation classes, which are commonly available at hospitals with maternity services and at local mothers' groups.

The breast prepares for lactation during pregnancy by enhancing the ductal system and developing lacteal cells that will produce the milk. From about sixteen weeks in gestation on, the breast is capable of making milk if the fetus is delivered. During pregnancy the placenta produces a prolactin-inhibiting hormone (PIH) that blocks the breast from responding to the abundant prolactin of pregnancy. Once the placenta is delivered, the PIH drops, and the breast responds to the hormones oxytocin and prolactin.

The key response of the breast following delivery is called the ejection or letdown reflex, prompted by two major hormones, oxytocin and prolactin. Oxytocin causes myoepithelial cells to contract. Thus when the baby stimulates the breast by suckling, a message is sent via the peripheral nervous system to the mother's brain and pituitary to release oxytocin, which in turn causes the myoepithelial cells that surround the alveoli and the ductal system to contract, ejecting the milk from the ducts. Suckling at the breast also stimulates the release of prolactin, the hormone that stimulates the lacteal cells to produce milk. Prolactin is not released unless the breast is stimulated. Oxytocin, however, may be released when the mother sees her baby or hears her baby cry or as a result of other stimulating sensory pathways.

It is recommended that the infant be put to breast as soon after delivery as is possible. The infant has been sucking and swallowing in utero, consuming considerable amniotic fluid, from about fourteen weeks gestation on, so he or she is ready to begin breast-feeding.

To put the infant to breast, the infant is held with his or her abdomen against the mother's and the infant looking directly at the breast. The mother supports the breast with her hand, keeping her fingers behind the areola and gently compressing it. The mother strokes the center of the infant's lower lip with the nipple. This stimulates the infant to open his or her mouth, extend his or her tongue, and draw the nipple and the areola into his or her mouth. The baby's tongue compresses the elongated nipple and areola against his or her hard palate. The peristaltic motion of the tongue stimulates the letdown reflex, and milk is released and swallowed. Infants should be fed when hungry, which is eight to twelve times a day initially. No other food or drink is necessary during exclusive breast-feeding for up to six months.

Bibliography

American Academy of Pediatrics Work Group on Breastfeeding. "Breastfeeding and the Use of Human Milk." Pediatrics 100 (1997): 1035.

Ball, Thomas M., and Anne L. Wright. "Health Care Costs of Formula-Feeding in the First Year of Life." Pediatrics 103 (1999): 870.

Biancuzzo, Marie. Breastfeeding the Newborn: Clinical Strategies for Nurses. St. Louis, Mo.: Mosby, 1999.

Horwood, L. John, and David M. Fergusson. "Breastfeeding and Later Cognitive and Academic Outcomes." Pediatrics 101 (1998): 39.

Huggins, Kathleen. The Nursing Mother's Companion. 4th ed. Boston: Harvard Common Press, 1999.

Institute of Medicine, Subcommittee on Nutrition during Lactation. Nutrition during Lactation. Washington, D.C.: National Academy Press, 1991.

Lawrence, Ruth A., and Robert M. Lawrence. Breastfeeding: AGuide for the Medical Profession. 5th ed. St. Louis, Mo.: Mosby, 1999.

Lucas, Alan, Ruth Morley, T. J. Cole, et al. "Breast Milk and Subsequent Intelligence Quotient in Children Born Preterm." Lancet 339 (1992): 261.

Newton, Niles. "Psychological Differences between Breast and Bottle Feeding." American Journal of Clinical Nutrition 24 (1971): 993.

United States Department of Health and Human Services. Healthy People 2010. Conference ed. in 2 vols. Washington, D.C.: U.S. Department of Health and Human Services, 2000.

—Ruth A. Lawrence

1. the secretion of milk by the mammary gland.
2. a complete period of milk secretion from about the time of parturition to that of weaning. See also lactate (def. 3).

1. the secretion of milk by the mammary glands.
2. the period of weeks or months during which the dam lactates.

• artificial l. — see lactation induction (below).
• current l. listing — a list of all the cows in a herd which are currently being milked.
• l. curves — daily milk yield plotted along one abscissa of a graph, days along the other. Used to monitor peak milk output and milk persistency, especially to assess nutritional management.
• early l. drop — an unexpected downturn in the lactation curve of a dairy cow in early lactation.
• l. failure — see agalactia.
• l. hormone — lactogenic hormone, or prolactin.
• inappropriate l. — see galactorrhea.
• l. induction — in nonpregnant cows by the administration of hormones, usually a combination of estradiol and progesterone.
• l. ketosis — see acetonemia.
• l. number — the number of times the cow has calved at the start of the current lactation. Used in the US and elsewhere by dairy herd improvement associations to divide cows in a dairy herd into groups for analysis.
• l.–pregnancy cycle — in dairy cows the cycle of the cow's year, commencing with calving and lactation onset, followed by conception, then drying off followed by calving again, all with impeccable timing, when aiming at a 365 day cycle.
• l. record — the total milk and components produced by a cow beginning on the day of calving and ending on the day the cow goes dry. For purposes of genetic comparision, 10-month (305-day) lactation records are the standard of the industry.
• projected 305-day l. — a calculation for predicting a cow's total yield in 305 days based on the information from a lactation in progress.

• Endocrine System and Glands - lactation: production of milk in mammary glands
• Reproduction and Development - lactation: production of milk by mother’s mammary glands

Kittens nursing

Lactation of pigs

Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, although it predates mammals.^[1] In humans the process of feeding milk is called breastfeeding or nursing.

In most species milk comes out of the mother's nipples; however, the platypus (a non-placental mammal) releases milk through ducts in its abdomen. In only one species of mammal, the Dayak fruit bat, is milk production a normal male function. In some other mammals, the male may produce milk as the result of a hormone imbalance. This phenomenon may also be observed in newborn infants as well (for instance witch's milk). Galactopoiesis is the maintenance of milk production. This stage requires prolactin (PRL) and oxytocin.

Contents 1 Purpose 2 Human lactation 2.1 Hormonal influences 2.1.1 Lactogenesis I 2.1.2 Lactogenesis II 2.1.3 Lactogenesis III 2.2 Milk ejection reflex 2.3 Afterpains 3 Lactation without pregnancy, induced lactation, relactation 4 Evolution 5 See also 6 References 7 External links

Purpose

The chief function of lactation is to provide nutrition and immune protection to the young after birth. In almost all mammals, lactation induces a period of infertility, which serves to provide the optimal birth spacing for survival of the offspring.^[2]

Human lactation

When the baby sucks its mother's breast, a hormone called oxytocin compels the milk to flow from the alveoli, through the ducts (milk canals) into the sacs (milk pools) behind the areola and then into the baby's mouth

Hormonal influences

From the twenty-fourth week of pregnancy (the second and third trimesters), a woman's body produces hormones that stimulate the growth of the milk duct system in the breasts:

• Progesterone—influences the growth in size of alveoli and lobes, high levels of progesterone inhibit lactation before birth. Progesterone levels drop after birth, this triggers the onset of copious milk production.^[3]
• Oestrogen—stimulates the milk duct system to grow and differentiate. Like progesterone high levels of oestrogen also inhibit lactation. Oestrogen levels also drop at delivery and remain low for the first several months of breastfeeding.^[3] It is recommended that breastfeeding mothers avoid oestrogen-based birth control methods, as a spike in estrogen levels may reduce a mother's milk supply.
• Prolactin—contributes to the increased growth and differentiation of the alveoli, also influences differentiation of ductal structures. High levels of prolactin during pregnancy and breastfeeding also increase insulin resistance, increase growth factor levels (IGF-1) and modify lipid metabolism in preparation for breastfeeding. During lactation prolactin is the main factor maintaining tight junctions of the ductal epithelium and regulating milk production through osmotic balance.
• Growth hormone is structurally very similar to prolactin and contributes to its galactopoietic function.
• ACTH and glucocorticoids have an important lactation inducing function in several animal species. ACTH is thought to contribute as it is structurally similar to prolactin. Glucocorticoids play a complex regulating role in the maintenance of tight junctions.
• TSH is a very important galactopoietic hormone, its levels are naturally increased during pregnancy.
• Oxytocin—contracts the smooth muscle of the uterus during and after birth, and during orgasm(s). After birth, oxytocin contracts the smooth muscle layer of band-like cells surrounding the alveoli to squeeze the newly-produced milk into the duct system. Oxytocin is necessary for the milk ejection reflex, or let-down to occur.
• Human placental lactogen (HPL)—From the second month of pregnancy, the placenta releases large amounts of HPL. This hormone appears to be instrumental in breast, nipple, and areola growth before birth.
• Follicle stimulating hormone (FSH)
• Luteinizing hormone (LH)

By the fifth or sixth month of pregnancy, the breasts are ready to produce milk. It is also possible to induce lactation without pregnancy.

Lactogenesis I

During the latter part of pregnancy, the woman's breasts enter into the Lactogenesis I stage. This is when the breasts make colostrum (see below), a thick, sometimes yellowish fluid. At this stage, high levels of progesterone inhibit most milk production. It is not a medical concern if a pregnant woman leaks any colostrum before her baby's birth, nor is it an indication of future milk production.

Lactogenesis II

At birth, prolactin levels remain high, while the delivery of the placenta results in a sudden drop in progesterone, estrogen, and HPL levels. This abrupt withdrawal of progesterone in the presence of high prolactin levels stimulates the copious milk production of Lactogenesis II.

When the breast is stimulated, prolactin levels in the blood rise, peak in about 45 minutes, and return to the pre-breastfeeding state about three hours later. The release of prolactin triggers the cells in the alveoli to make milk. Prolactin also transfers to the breast milk. Some research indicates that prolactin in milk is greater at times of higher milk production, and lower when breasts are fuller, and that the highest levels tend to occur between 2 a.m. and 6 a.m.^[4]

Other hormones—notably insulin, thyroxine, and cortisol—are also involved, but their roles are not yet well understood. Although biochemical markers indicate that Lactogenesis II begins about 30–40 hours after birth, mothers do not typically begin feeling increased breast fullness (the sensation of milk "coming in the breast") until 50–73 hours (2–3 days) after birth.

Colostrum is the first milk a breastfed baby receives. It contains higher amounts of white blood cells and antibodies than mature milk, and is especially high in immunoglobulin A (IgA), which coats the lining of the baby's immature intestines, and helps to prevent pathogens from invading the baby's system. Secretory IgA also helps prevent food allergies.^[5] Over the first two weeks after the birth, colostrum production slowly gives way to mature breast milk.^[3]

Lactogenesis III

The hormonal endocrine control system drives milk production during pregnancy and the first few days after the birth. When the milk supply is more firmly established, autocrine (or local) control system begins. This stage is called Lactogenesis III

During this stage, the more that milk is removed from the breasts, the more the breast will produce milk.^[6][7] Research also suggests that draining the breasts more fully also increases the rate of milk production.^[8] Thus the milk supply is strongly influenced by how often the baby feeds and how well it is able to transfer milk from the breast. Low supply can often be traced to:

• not feeding or pumping often enough
• inability of the infant to transfer milk effectively caused by, among other things:
  • jaw or mouth structure deficits
  • poor latching technique
• rare maternal endocrine disorders
• hypoplastic breast tissue
• a metabolic or digestive inability in the infant, making it unable to digest the milk it receives
• inadequate calorie intake or malnutrition of the mother

Milk ejection reflex

Leaking milk from a human breast.

Milk ejection is initiated in the mother's breast by the act of suckling by the baby. The let-down reflex is not always consistent, especially at first. Once a woman is conditioned to nursing, let-down can be triggered by a variety of stimuli, including the sound of any baby. Even thinking about breastfeeding can stimulate this reflex, causing unwanted leakage, or both breasts may give out milk when an infant is feeding from one breast. However, this and other problems often settle after two weeks of feeding. Stress or anxiety can cause difficulties with breastfeeding. The release of the hormone oxytocin leads to the milk ejection or let-down reflex. Oxytocin stimulates the muscles surrounding the breast to squeeze out the milk. Breastfeeding mothers describe the sensation differently. Some feel a slight tingling, others feel immense amounts of pressure or slight pain/discomfort, and still others do not feel anything different.

A poor milk ejection reflex can be due to sore or cracked nipples, separation from the infant, a history of breast surgery, or tissue damage from prior breast trauma. If a mother has trouble breastfeeding, different methods of assisting the milk ejection reflex may help. These include feeding in a familiar and comfortable location, massage of the breast or back, or warming the breast with a cloth or shower.

Afterpains

A surge of oxytocin also causes the uterus to contract. During breastfeeding, mothers may feel these contractions as afterpains. These may range from period-like cramps to strong labour-like contractions and can be more severe with second and subsequent babies. Some women's breasts also become dry and chapped and even crack open and bleed while breast feeding.This has many different causes. To treat painful nipples treating the underling cause is the best action to take. In the mean time gelpads and/or rubbing lanolin on the nipples and areola can reduce associated pain.^{[9] [10]}

Lactation without pregnancy, induced lactation, relactation

In humans induced lactation and relactation has been observed frequently in primitive cultures and demonstrated with varying success in adoptive mothers. It appears plausible that the possibility of induction of lactation in women (or females of other species) who are not biological mothers does confer an evolutionary advantage especially in groups with high maternal mortality and tight social bonds.^{[11] [12]} The phenomenon has been also observed in most primates, some lemurs and dwarf mongooses.^[13][14]

Lactation can be induced in humans by a combination of physical and psychological stimulation, by drugs, or by a combination of those methods.^[15] Some couples may stimulate lactation outside of pregnancy for sexual purposes.

Rare accounts of male lactation (as distinct from galactorrhea) exist in historical medical and anthropological literature, although the phenomenon has not been confirmed by more recent literature.^[16]

Evolution

A 5 day old mouse pup, suckling on an anesthetized, lactating female

Darwin correctly recognised that mammary glands developed from cutaneous glands and hypothesized that they evolved from glands in brood pouches of fish where they provided nourishment for eggs.^[1] The later aspect of his hypothesis has not been confirmed, but recently the same mechanism has been postulated for early synapsids.^[17] Instead the discus fish (Symphysodon aequifasciata) became known for (biparentally) feeding their offspring by epidermal mucus secretion.^[18] A closer look reveals that similar to most mammals the secretion of the nourishing fluid may be controlled by prolactin.^[19]

Later therapsids such as cynodonts appear to have secreted complex, nutrient-rich milk. This brought them evolutionary advantage by allowing a decline in egg size.^[17]

During early evolution of lactation the secretion was through pilosebaceous glands, and mammary hairs transported the nourishing fluids to the eggs or youngs. Later the development of the mammary patch rendered mammary hairs obsolete. ^[17]

Other well known example of nourishing young with secretions of glands is the crop milk of pigeons. Like in mammals and disc fish this also appears closely related to prolactin.^[20] Other birds such as flamingos and penguins are utilizing similar feeding techniques.^[21]

Further information: Evolution of mammals#Milk production (lactation)

Further information: Mammary gland#Evolution

See also

• Lactation room
• Galactogogue
• Milk line
• Udder
• Breastfeeding

References

1. ^ ^{a b} Capuco, A. V.; Akers, R. M. (2009). "The origin and evolution of lactation". Journal of Biology 8 (4): 37. doi:10.1186/jbiol139. PMC 2688910. PMID 19439024. //www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2688910.  edit
2. ^ McNeilly, A. S. (1997). "Lactation and fertility". Journal of Mammary Gland Biology and Neoplasia 2 (3): 291–298. doi:10.1023/A:1026340606252. PMID 10882312. 
3. ^ ^{a b c} Mohrbacher, Nancy; Stock, Julie (2003). The Breastfeeding Answer Book (3rd ed. (revised) ed.). La Leche League International. ISBN 0-912500-92-1. 
4. ^ Cregan M, Mitoulas L, Hartmann P (2002). "Milk prolactin, feed volume and duration between feeds in women breastfeeding their full-term infants over a 24 h period". Exp Physiol 87 (2): 207–14. doi:10.1113/eph8702327. PMID 11856965. 
5. ^ Sears, Martha; Sears, William (2000). The Breastfeeding Book. Little, Brown. ISBN [[Special:BookSources/978-0-316-77924-5|978-0-316-77924-5]]. 
6. ^ deCarvalho M, Anderson D, Giangreco A, Pittard W (1985). "Frequency of milk expression and milk production by mothers of non-nursing premature neonates". Am J Dis Child 139 (5): 483–5. PMID 3984973. 
7. ^ Hopkinson J, Schanler R, Garza C (1988). "Milk production by mothers of premature infants". Pediatrics 81 (6): 815–20. PMID 3368280. 
8. ^ Daly S, Owens R, Hartmann P (1993). "The short-term synthesis and infant-regulated removal of milk in lactating women". Exp Physiol 78 (2): 209–20. PMID 8471241. 
9. ^ Breastfeeding Answers Made Simple, Nancy Mohrbacher, IBCLC, FILCA
10. ^ Fray, Kathy (2005). Oh Baby...Birth, Babies & Motherhood Uncensored. Random House NZ. ISBN 1-86941-713-5. 
11. ^ Sobrinho, L. (2003). "Prolactin, psychological stress and environment in humans: adaptation and maladaptation". Pituitary 6 (1): 35–39. doi:10.1023/A:1026229810876. PMID 14674722.  edit
12. ^ Bose, C.; D'ercole, A.; Lester, A.; Hunter, R.; Barrett, J. (1981). "Relactation by mothers of sick and premature infants". Pediatrics 67 (4): 565–569. PMID 6789296.  edit
13. ^ König, B. (1997). "Cooperative care of young in mammals". Die Naturwissenschaften 84 (3): 95–104. Bibcode 1997NW.....84...95K. doi:10.1007/s001140050356. PMID 9112240.  edit
14. ^ Creel, S. R.; Monfort, S. L.; Wildt, D. E.; Waser, P. M. (1991). "Spontaneous lactation is an adaptive result of pseudopregnancy". Nature 351 (6328): 660–662. Bibcode 1991Natur.351..660C. doi:10.1038/351660a0. PMID 2052092.  edit
15. ^ "Relactation: an effective intervention to promote exclusive breastfeeding". Journal of tropical pediatrics 43 (4): 213–6. 1997. PMID 9283123.  edit
16. ^ http://www.scientificamerican.com/article.cfm?id=strange-but-true-males-can-lactate&sc=rss
17. ^ ^{a b c} Oftedal, OT (2002). "The mammary gland and its origin during synapsid evolution". Journal of Mammary Gland Biology and Neoplasia 7 (3): 225–52. doi:10.1023/A:1022896515287. PMID 12751889.  edit
18. ^ Chong, K.; Joshi, S.; Jin, L. T.; Shu-Chien, A. C. (2006). "Proteomics profiling of epidermal mucus secretion of a cichlid (Symphysodon aequifasciata) demonstrating parental care behavior". Proteomics 6 (7): 2251. doi:10.1002/pmic.200500591. PMID 16385477.  edit
19. ^ Khong, H. K.; Kuah, M. K.; Jaya-Ram, A.; Shu-Chien, A. C. (2009). "Prolactin receptor mRNA is upregulated in discus fish (Symphysodon aequifasciata) skin during parental phase". Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology 153: 18. doi:10.1016/j.cbpb.2009.01.005.  edit
20. ^ Horseman, N. D.; Buntin, J. D. (1995). "Regulation of Pigeon Cropmilk Secretion and Parental Behaviors by Prolactin". Annual Review of Nutrition 15: 213. doi:10.1146/annurev.nu.15.070195.001241. PMID 8527218.  edit
21. ^ http://www.stanford.edu/group/stanfordbirds/text/essays/Bird_Milk.html

External links

• How mammals lost their egg yolks—Did mammals develop nutritional milk before or after they abandoned yolky eggs? (New Scientist, 18 March 2008)

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