birth defect

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Structural abnormalities of the body that develop during embryogenesis and the fetal period; also called birth defects. Children with significant birth defects need more medical care than other children do, require more frequent hospitalizations, need community support services, and often require special education programs. Among children, over half of all visits to subspecialty medical clinics and admissions to hospitals are for treatment of disorders resulting from errors in embryonic development, chromosomal abnormalities, and genetic and familial disorders. Two-thirds of the deaths of infants and children in pediatric hospitals in developed countries are caused by underlying congenital anomalies.

Screening programs are available to identify fetuses and newborns likely to have disorders such as congenital malformations and genetic diseases. Examples of screening include amniocentesis to detect fetal chromosomal abnormalities in mothers over 35 years of age; measurement of maternal serum alpha-fetoprotein levels at 15–16 weeks' gestation to help identify fetuses with certain malformations; and screening of newborns for phenylketonuria, sickle cell disease, thalassemia, galactosemia, and congenital hypothyroidism. See also Alpha fetoprotein; Phenylketonuria; Sickle cell disease.

Birth defects can be caused by genetic factors, exposure to malformation-causing agents (teratogens), or a combination of both. Dysmorphology is the area of medicine and science concerned with the cause of congenital anomalies resulting from errors in embryonic development (dysmorphogenesis). The study of normal and abnormal embryonic development allows identification of the latest time in embryogenesis when a malformation could occur. Examples of such times include 28 days of gestation for neural tube defects, 36 days for cleft lip, and 10 weeks for cleft palate.

To facilitate study, birth defects are divided into malformations, disruptions, deformations, and dysplasias. Malformations are structural defects that are caused by primary errors in morphogenesis. They are classified as major and minor. Major malformations require medical or surgical intervention or are of substantial cosmetic importance. Minor malformations do not require such treatment or do not greatly affect appearance. Disruptions are structural defects resulting from interruption of normal morphogenesis, with consequent destruction of previously existing structures and incomplete development of tissues. Deformations are congenital anomalies resulting from external compression of a normally formed part of the fetus. Dysplasias are disorders that result from an abnormal organization of cells into tissues; the morphological result is called dyshistogenesis.

Some individuals have various major and minor congenital anomalies that together form a recognizable pattern, called a syndrome. Syndromes can have both genetic and environmental causes.

Tetratology is the study of the effect of environmental agents on the developing embryo and fetus. Teratogens are agents that interfere with normal embryonic development. They can cause miscarriages, retard prenatal growth, and produce congenital anomalies or mental retardation. There are five general groups of teratogens: (1) infectious diseases and agents; (2) physical agents, such as radiation; (3) drugs and chemical agents; (4) maternal metabolic and genetic factors, such as diabetes; and (5) paternal factors, although rare.

Treatment of congenital anomalies is specific for each individual. Individuals with severe or numerous abnormalities usually require multidisciplinary treatment, including such measures as medical management, surgical correction, nursing care, special diets, rehabilitation, prosthetic devices, special education, and community support. Measures that help reduce the risk of having a child with congenital anomalies include avoidance of teratogenic exposures, medical treatment of maternal illnesses, good nutrition, and routine obstetrical care. See also Pregnancy.

This term refers broadly to anything that is wrong with an infant at birth, whether it be a true congenital abnormality resulting from maldevelopment of the fetus in utero (from either inherent genetic or extraneous causes) or damage to the normally-developed fetus by infection or injury before or during birth. The term may cover both those defects which are evident at the time of birth and also those which reveal themselves later, including inborn errors of metabolism.

— Stuart Judge

See antenatal development; birthmark; cleft lip and palate; congenital abnormalities.

A congenital anomaly may be viewed as a physical, metabolic, or anatomic deviation from the normal pattern of development that is apparent at birth or detected during the first year of life. Under this definition, Mendelian genetic disorders (e.g., phenylketonuria), chromosomal abnormalities (e.g., Down syndrome), tumors (e.g., Wilms' tumor), infections (e.g., rubella, toxoplasmosis, herpes virus, cytomegalovirus, HIV, and syphilis), exposure to teratogenic agents (e.g., cocaine, tobacco, or alcohol), maternal disease (e.g., maternally transmitted autoantibodies, phenylketonuria), and pure bad luck or accident (e.g., a twisted umbilical cord) can all contribute to the development of a congenital anomaly. It is important to determine which of these predisposing conditions have led to the anomaly, because knowledge of the etiologic agent or agents influence not only therapy, but also prevention in the case of future pregnancies.

In the United States in 1998, of nearly 4 million live births, just over 45,000 babies (1.15 percent of births) had congenital anomalies of significant enough severity to be recorded on their birth certificates. Musculoskeletal anomalies (e.g., cleft lip/palate, polydactyly, clubfoot) were most common (465 per 100,000 live births), followed by cardiovascular and respiratory malformations (250 per 100,000 live births), urogenital malformations (e.g., malformed genitalia, renal agenesis; 193 per 100,000 live births), central nervous system malformations (e.g., anencephaly, spina bifida, hydrocephalus, microcephalus; 83 per 100,000 live births), gastrointestinal malformations (e.g., rectal atresia/stenosis, tracheo-esophageal fistula, omphalocoele; 83 per 100,000 live births), and multiple malformations attributable to chromosomal anomalies (77 per 100,000 live births).

Prevention is the best approach to congenital anomalies. A teratogen can be defined as an agent or factor (e.g., infectious agents, physical agents such as radiation and heat, drug and chemical agents, and maternal metabolic and genetic factors) that can produce abnormalities of form and function in an exposed fetus. As a general rule, organ systems are created during the first trimester of life, structured during the second trimester, and undergo maturation in the third trimester. Thus, teratogens tend to exercise their most destructive effect during the first and second trimesters, underscoring the importance of avoiding exposures to known teratogens from the point a decision is made to consider pregnancy. Prophylaxis can also be practiced, for example, by fortifying the diet with folic acid to reduce the risk of neural tube defects.

Abnormal development of major organ systems is readily apparent before the end of the second trimester, making examination of the fetus by ultrasound the simplest form of screening. Some conditions such as obstruction of the urinary tract, are treatable in utero. Evaluation for specific disorders is also available for mothers at risk as a result of genetic background, ethnicity, age, history of exposure, or other routine screening tests. The potential benefits from a given procedure must be balanced against the expected risk. For example, the vast majority of babies with Down syndrome are born to mothers between the age of twenty and thirty; however, the risk of having a baby with Down syndrome begins to increase exponentially after age thirty. Definitive diagnostic procedures, such as chorionic villous sampling and amniocentesis, carry the risk of abortion, hence most physicians discourage these procedures for younger women, where the risk of complications is greater than the prevalence of the suspected anomaly, if the parents have already decided that an induced abortion is out of the question.

Gross abnormalities are obvious at birth, whereas many metabolic abnormalities are not immediately apparent and represent a significant, and possibly preventable, hazard to the health and well-being of the patient. In general, neonatal screening is advisable when the incidence of the disease is sufficient to warrant mass screening of the population; when the test is sufficiently sensitive to detect the disease while specific enough to minimize the stress incurred in ruling out the diagnosis; and when the disease is not only treatable, but early diagnosis is critical. Examples include phenylketonuria and congenital hypothyroidism, both of which lead to preventable and relatively silent forms of mental retardation and where delay in diagnosis can lead to irreparable loss of intelligence. In contrast, galactosemia may also result in mental retardation, but the gastrointestinal distress experienced by the infant typically leads to early diagnosis, and long-term results of treatment have been disappointing—the IQ is low in many patients despite early and seemingly adequate therapy.

(SEE ALSO: Birth Certificates; Birthrate; Genes; Genetic Disorders; Genetics and Health; Maternal and Child Health; Medical Genetics; Newborn Screening; Perinatology; Phenylketonuria; Pregnancy; Prenatal Care; Teratogens)

Bibliography

Corcoran, J. (1998). "What Are the Molecular Mechanisms of Neural Tube Defects?" Bioessays 20 (1):6–8.

McKusick, V. A. (1998). Mendelian Inheritance in Man. A Catalog of Human Genes and Genetic Disorders, 12th edition. Baltimore, MD: Johns Hopkins University Press.

Moyer, A.; Brown, B.; Gates, E.; Daniels, M.; Brown, H. D.; and Kuppermann, M. (1999). "Decisions about Prenatal Testing for Chromosomal Disorders: Perceptions of a Diverse Group of Pregnant Women." Journal of Women's Health & Gender-Based Medicine 8 (4):521–531.

Rimoin, D. L.; Connor, J. M.; and Pyeritz, R. E. (1997). Emery and Rimoin's Principles and Practice of Medical Genetics, 3rd edition. New York: Churchill Livingstone.

Ventura, S. J.; Martin, J. A.; Curtin, S. C.; Mathews, T. J.; and Park, M. M. (2000). "Births: Final Data for 1998." National Vital Statistics Reports 48 (3): 1–100.

— HARRY W. SCHROEDER, JR.

A birth defect is an anomaly that is congenital, or present from birth. Birth defects are the leading cause of infant mortality, causing 22 percent of all infant deaths. Approximately 3 to 4 percent of all live births are affected by a birth defect; the causes of most of them are unknown. Some birth defects are considered to be physical, while others are thought of as functional. Physical birth defects result in the malformation of a physical organ or limb, whereas functional birth defects are those that cause primarily functional, rather than physical, problems. Functional birth defects include mental retardation, congenital hearing loss, early-onset vision impairment, and numerous other health concerns. They may be caused by single gene mutations, or they may be due to polygenic or multifactorial inheritance.

Birth defects may be found in isolation or they may occur in combination in one child, as part of a larger syndrome. A syndrome is usually defined as the presence of three or more birth defects due to one underlying cause, that characterize a particular disease or condition. An example of this is Down syndrome.

Various Causes, Various Treatments

There are several etiologies of birth defects, including single gene mutations, polygenic and multifactorial conditions, chromosomal abnormalities, and teratogens, which cause growth or developmental abnormalities. These etiologies may cause both physical and functional birth defects. The diagnosis, treatment, and management of birth defects often involves a team of professionals and specialists. Among these specialists are clinical geneticists, medical geneticists, and genetic counselors. These are all genetic service providers who are trained to help make a diagnosis and identify whether a birth defect is isolated or part of a syndrome. An accurate diagnosis is of utmost importance in being able to treat the condition and anticipate any future health concerns that may arise.

Table 1

Single-Gene Mutations

Single-gene mutations are defects or changes in genes that may be passed on from generation to generation. The most common forms of inheritance include autosomal dominant, autosomal recessive, X-linked recessive, and new dominant mutations. Single-gene mutations may result in both structural and functional birth defects. These defects may be found in isolation or as part of a known syndrome and may display phenotypic variation, in which the same mutant gene leads to variable clinical problems. This often occurs with dominant mutations such as Marfan syndrome. A genetic disorder may also display reduced penetrance, wherein not everyone with the genetic mutation will have the disorder or trait. These are also known as "silent" disorders.

Autosomal Dominant Disorders

An example of an autosomal dominant disorder is achondroplasia, the most common form of short-limbed dwarfism in humans. Achondroplasia displays complete penetrance (everyone with the genetic defect also has the disorder), and it occurs in 1 out of 25,000 births. Most cases are sporadic rather than inherited.

Achondroplasia is a growth disorder caused by a mutation of the gene that encodes the fibroblast growth factor receptor 3 (FGFR3), and it is characterized by short limbs, malformed hands, a disproportionately large head, and abnormal facial features. Medical problems are due to abnormally configured bones and related structures, leading to hydrocephalus, problems of the spine, frequent sinus and ear infections, and orthopedic problems. If one parent is affected with achondroplasia, there is a 50 percent risk that an offspring will also be affected. If both parents are affected, there is a 25 percent chance that an offspring will inherit two gene copies and develop severe, life-threatening features. Two known mutations in FGFR3 account for 98 percent of all achondroplasia cases. This makes early identification, even prenatal diagnosis, relatively easy.

Another autosomal dominant condition is Marfan syndrome, which results from a defect in the synthesis, secretion, or utilization of the protein fibrillin, an important component of connective tissue throughout the body. The gene for Marfan syndrome is fibrillin 1 (FBR1). Marfan syndrome features are variable, including cardiovascular, skeletal, and ocular defects.

Marfan syndrome's most serious medical complication is the risk of sudden death from aortic dissection, a tear in the inner wall of the major artery leading from the heart. Approximately 75 percent of individuals with Marfan syndrome have a family history of the disease, with the rest occurring as new mutations. Because the syndrome is autosomally dominant, affected indviduals have a 50 percent risk of passing the mutated gene to their off-spring. The condition has full penetrance; therefore, all individuals who inherit this mutation will express some features of Marfan syndrome.

Functional Birth Defects

Fragile X syndrome is the most common cause of inherited mental retardation, occurring in one out of 1,000 births. It is caused by expansion of a "triplet repeat" section of nucleotides in the FMR-1 gene on the X chromosome. Triplet repeats are three-base-pair sequences in a gene that are abnormally repeated, sometimes dozens or even hundreds of times, causing abnormal protein sequence and structure. Because it is carried on the X chromosome, it affects males more often than females. In males, an FMR-1 gene with greater than 200 repeats is always associated with the syndrome. Inactivated FMR-1 gene causes impaired mental function. The FMR-1 protein is thought to help shape the connections between neurons that underlie learning and memory. Affected individuals may also have large testes, abnormal facial features, seizures, and emotional and behavior problems. DNA testing allows for detection of carriers as well as affected individuals, enabling the use of genetic counseling and prenatal testing.

Another common functional genetic disorder is hearing loss, which can result from a defect in any one of more than fifty different genes. One gene, connexin 26, may be responsible for a large portion of inherited hearing loss. Some forms of congenital hearing loss may be due to prenatal exposure to infectious agents such as rubella. Genetic screening or screening for hearing loss at birth may be the most important test for hearing impairment yet to be developed, as early recognition and treatment can lead to dramatic improvements in hearing and, consequently, in the development of language in early childhood.

Other genetic disorders that cause functional birth defects include those involved in various aspects of the immune system. The most severe form is severe combined immune deficiency (SCID), in which a major type of immune cell, the lymphocyte, is absent. People with SCID suffer life-threatening infections beginning in infancy and may require complete physical isolation. This was the case for David Vetter, who became known to the world as the "bubble boy."

Multifactorial and Polygenic Inheritance

Many traits and diseases are caused by the interaction of inherited genes and the environment. These are known as "multifactorial" traits. While all genes interact with the environment, the impact of the environment in multifactorial traits and diseases is usually greater than in single-gene traits and diseases. Prenatal environmental influences are inevitably filtered through the maternal-placental system and include factors such as infections, drugs, tobacco or alcohol use, diabetes, and industrial toxins.

Polygenic traits and diseases are due to the cumulative effect of multiple genes, working together. Many congenital birth defects are thought to be multifactorial, such as pyloric stenosis (narrowing of the passage from stomach to intestine), cleft lip and palate, clubfoot, and neural tube defects. When found as isolated birth defects, these conditions are thought to be explained by a "multifactorial threshold model."

The multifactorial threshold model assumes the gene defects for multifactorial traits are normally distributed within the population. This means that almost everyone has some genes involved with these conditions, with most individuals having too few of them to cause disease. Individuals will not become affected with the condition unless they have a genetic liability that is significant enough to push them past the threshold, moving them out of the unaffected range and into the affected range (Figure 1).

Examples of Multifactorial and Polygenic Effects

Cleft lip with or without cleft palate (CL/P) is a heterogeneous disorder (those children affected may have somewhat different abnormalities) occurring in 1 out of 1,000 births. Some CL/P cases occur as isolated birth defects, while others occur as part of a larger syndrome. The majority of CL/P cases are associated with multifactorial inheritance. The risk to relatives of affected individuals can be anywhere from 0.5 to 15 percent, depending on the severity of the clefting and the degree of relationship to the affected individual, with risks highest for first-degree relatives. Some unique cases of CL/P may be associated with genetic syndromes that are due to single-gene mutations or chromosomal abnormalities.

Clubfoot is another primarily multifactorial defect and occurs in 1 out of 10,000 Caucasian newborns. The estimated risk to relatives of inheriting this defect is between 2 to 20 percent, depending upon the family history. Clubfoot can also have genetic causes such as chromosomal abnormalities or single-gene disorders, or it may have an environmental origin, such as problems caused by amniotic fluid or structural abnormalities of the uterus that restrict fetal growth and mobility. Clubfoot can also be due to autosomal recessive as well as autosomal dominant inheritance, and it may also occur as part of a larger syndrome.

Another class of multifactorial disorders is known as neural tube defects (NTDs). The neural tube is the embryonic structure that develops into the brain and spinal cord. Failure of the neural tube to close, which normally occurs during in the fourth week of gestation, results in an NTD, usually spina bifida or anencephaly. Spina bifida ("open spine") is a defect of the spine. The most common form of spina bifida causes some degree of leg paralysis, impaired bladder and bowel control, and sometimes mental retardation. Anencephaly is a rapidly fatal condition in which a baby is born with a severely underdeveloped brain and skull.

While most NTDs are inherited as multifactorial disorders, a few result from single-gene disorders, chromosomal abnormalities, or teratogens. NTDs currently have an incidence of 1 per 2,000 births. This rate has fallen dramatically over the past thirty years, due to the remarkable effects of NTD-prevention efforts. Maternal deficiency in folate (a B vitamin) greatly increases the risk of NTDs, but taking multivitamins containing folic acid before conception and early in pregnancy is highly effective in preventing these disorders. High doses of folic acid are needed to help protect the fetuses of women with pregnancies previously affected by NTDs, and for those who need to take certain medications that interfere with folate metabolism.

Chromosome Disorders

Humans normally have twenty-two pairs of autosomes and two sex chromosomes, XX or XY, making forty-six chromosomes in total. Chromosomal abnormalities occur in about 0.5 percent of all live births and are usually due to an abnormal number of chromosomes. These are nearly always an addition or deletion of a single autosome or sex chromosome in a pair. One extra copy of a chromosome is called a trisomy, while one missing copy of a chromosome is called a monosomy. Sometimes only a segment of a chromosome is duplicated or lost.

Chromosomal disorders are diagnosed by karyotype analysis and can be done on adults by testing blood, skin, or other tissue. Karyotypes can also be performed on a fetus through specialized testing such as amniocentesis. Prenatal maternal blood tests are routinely used to screen for some trisomies, though accurate diagnosis requires fetal karyotyping.

Chromosomal abnormalities can occur in offspring of mothers of all ages, but the frequencies of these disorders increase with maternal age, rising exponentially after the maternal age of thirty-five. Advanced paternal age has far less impact. Chromosomal abnormalities can result in either physical or functional birth defects. The severity of these birth defects is highly variable and depends upon the exact chromosome problem.

Chromosomal defects include such problems as Down syndrome, Kline-felter's syndrome, and Turner's syndrome. The majority of Down syndrome cases are due to an extra chromosome 21. Trisomy 21 usually occurs as an isolated event within a family. It results in characteristic facial features, lax muscle tone, cardiac and intestinal anomalies, and mild or moderate mental retardation. Additional medical complications may also include recurrent ear and respiratory tract infections, vision problems, hearing difficulties, and short stature.

Klinefelter's syndrome is a sex chromosome abnormality that occurs in 1 of 600 males, with a karyotype of 47. Individuals with Klinefelter's syndrome possess an extra X chromosome: XXY. Clinical characteristics are variable and include some learning and developmental disabilities, hypogonadism, small testes, and gynecomastia occuring in puberty. The condition can be managed by administering testosterone supplements beginning in adolescence. As with some other sex chromosome abnormalities, adults with Klinefelter's syndrome are usually infertile.

Turner's syndrome is another sex chromosome disorder, with a karyotype of 45. In this condition, one X chromosome is missing. Turner's syndrome occurs in one out of 4,000 live births. Most females with Turner's syndrome are short and have webbing of the neck, a broad chest, and a lack of ovarian development, with a consequent lack of pubertal development and infertility. Female hormone therapy is often used to induce breast development and menstruation. The majority of conceptions resulting in a fetus with this condition end in a miscarriage, as Turner's syndrome is highly lethal in early fetal development.

Teratogen Exposure

A teratogen is any agent that can cause birth defects if a fetus is exposed to it. Teratogens are usually drugs or infectious agents such as bacteria or viruses, and can affect a fetus from as early as the first few weeks after conception through the second trimester. For this reason, ideally, women should avoid all medications during pregnancy. However, in some circumstances there are medical risks and benefits that must be weighed, particularly if a medication is important to the health of the prospective mother.

To properly assess the potential danger of a teratogen, information is required about its effect on embryonic development, its ease of passage across the placenta, and the dosage and timing of fetal exposure to the teratogen. There are, however, limitations on the ability of doctors to predict the risk of birth defects arising from fetal exposure to any particular drug because of the lack of information on the effects of multiple medication use and possible drug interactions, the inability to control for other exposures women may have during pregnancy, and the unique genetic susceptibilities of each person. In addition, there are limited clinical studies that address this problem.

Risks for birth defects or adverse pregnancy outcomes associated with any type of exposure are in addition to the 3 percent background risk for birth defects in all pregnancies. There is no evidence linking paternal exposures to teratogenicity for the developing fetus, though exposure to some agents can reduce male fertility.

One well-documented teratogen is the drug thalidomide, which was taken by tens of thousands of women in the 1950s and early 1960s to treat nausea during pregnancy, before its potent teratogenic effects were recognized. Even a single dose caused severe birth defects, including amelia (absence of limbs), phocomelia (short limbs), incomplete or absent bone growth, ear and eye abnormalities, congenital heart defects, and others.

Another potent teratogen is a substance called isotretinoin, marketed under the brand name of Accutane, used to treat severe acne. Birth defects following prenatal exposure include serious central nervous system defects such as hydrocephalus, microcephaly, and mental retardation, as well as cleft lip and palate and anomalies of cardiovascular, limb, eye, and other systems. For women who have taken this substance, it is recommended to delay pregnancy at least one month after they have stopped using it.

Like medications, "recreational" drugs such as alcohol and cocaine can act as teratogenic agents. Fetal alcohol syndrome (FAS) and fetal alcohol effects (FAE) are the most common, completely preventable, and potentially devastating disorders caused by alcohol use during pregnancy. FAS is one of the leading causes of mental disabilities in children. Fetal exposure to alcohol creates additional problems in children born with FAS/FAE, such as characteristic facial features, growth retardation, central nervous system difficulties, learning disabilities, and behavioral problems. No amount of alcohol is thought to be safe during pregnancy; however, some of its effects may be prevented by stopping the exposure during or shortly after the first trimester.

Cocaine use during pregnancy is known to increase the risk of miscarriages and premature labor and delivery. Disturbances in the behavior of exposed newborns have been reported, such as irritability, irregular sleeping patterns, muscular rigidity, and poor feeding. Some birth defects associated with the use of this drug include urinary and genital malformations, as well as defects of the limbs, intestines, and the skull.

Conditions arising from infectious teratogenic agents include toxoplasmosis, syphilis, and rubella. In each of these cases, the mother is exposed to the infectious agent, then transmits it to the fetus. In toxoplasmosis, the parasite Toxoplasma gondii can be transmitted from cats to humans through contact with cat feces (cleaning litter box or gardening), or through consumption of undercooked meats, poorly washed fruits and vegetables, goat's milk, or raw eggs. Mother-to-fetus transmission is more likely if maternal infection occurs in the last few weeks before delivery, but early fetal exposure is generally associated with greater severity of defects in the child. Overall, 20 to 30 percent of untreated, infected newborns have birth defects, including seizures, microcephaly, and other severe effects on the nervous system. Treatment of the mother with antibiotics during pregnancy is safe for the fetus, and significantly reduces the likelihood of fetal infection.

Syphilis is an infection caused by the spirochete Treponema pallidum. This bacterium crosses the placenta and may result in fetal infection. If untreated, the pregnancy may end in miscarriage, stillbirth, or neonatal death. Signs of congenital infection include jaundice, joint swelling, rash, anemia, and characteristic defects of bone and teeth. Maternal treatment of this condition may help prevent the transmission to the fetus and its ill effects.

Rubella is the scientific name for the disease commonly known as German measles. Congenital rubella syndrome (CRS) results from the exposure of an unprotected pregnant woman to the rubella virus, and can lead to major birth defects, including serious malformations of the heart, blindness, deafness, and mental retardation. CRS has been virtually eradicated in the United States because of the near-universal vaccination against rubella, now part of the standard childhood immunizations program. Unfortunately, this vastly improved situation is not as common in much of the rest of the world.

Maternal Conditions

Birth defects can also result from physical conditions affecting the health of the mother. One common maternal condition associated with birth defects is diabetes mellitus, a multifactorial disorder. Mothers with diabetes have a two-to three-fold times greater risk of having a child with birth defects than the general population if their condition is not well controlled. However, good glucose control has been shown to correlate with a decreased risk of congenital malformations. Characteristic diabetic malformations include cardiovascular, craniofacial, genitourinary, gastrointestinal, and neurological abnormalities. The risk that the child born of a mother with diabetes mellitis will also develop diabetes as an adult is 1 to 3 percent.

Another maternal condition giving rise to birth defects is maternal phenylketonuria (PKU). This is an autosomal recessive disorder in which an enzyme called phenylalanine hydroxylase is defective. This enzyme normally converts a substance in the blood called phenylalanine to another substance called tyrosine. As a result, phenylalanine levels are high, resulting in mental retardation, microencephaly, growth retardation, cardiac problems, seizures, vomiting, and hyperactivity. Other traits associated with PKU are fair hair and skin and blue eyes. PKU can be effectively managed through changes in the diet, and women who have appropriately managed their diet can have pregnancies with healthy offspring.

Bibliography

Batshaw, Mark L. When Your Child Has a Disability: The Complete Sourcebook of Daily and Medical Care. Baltimore, MD: Paul H. Brooks Publishing, 2001.

"Leading Categories of Birth Defects." March of Dimes Perinatal Data Center, 2000.

Internet Resource

National Organization of Rare Disorders. http://www.rarediseases.org.

—Nancy S. Green and Terri Creeden

• Defects and Disabilities - birth defect: disability or physical deformity present at birth, either inherited or acquired, that stems from problems during fetal period; congenital defect
• Pregnancy and Birth - birth defect: physical defect present at birth that may be inherited or environmentally induced during pregnancy

Congenital disorder
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MeSH	D009358

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v d e

A congenital disorder, or congenital disease, is a condition existing at birth and often before birth, or that develops during the first month of life (neonatal disease), regardless of causation. Of these diseases, those characterized by structural deformities are termed "congenital anomalies"; that is a different concept (MeSH) which involves defects in or damage to a developing fetus.

A congenital disorder may be the result of genetic abnormalities, the intrauterine (uterus) environment, errors of morphogenesis, infection, or a chromosomal abnormality. The outcome of the disorder will depend on complex interactions between the pre-natal deficit and the post-natal environment.^[1] Animal studies indicate that the mother's (and possibly the father's) diet, vitamin intake, and glucose levels prior to ovulation and conception have long-term effects on fetal growth and adolescent and adult disease.^[2] Congenital disorders vary widely in causation and abnormalities. Any substance that causes birth defects is known as a teratogen.

The older term congenital^[3] disorder does not necessarily refer to a genetic disorder despite the similarity of the words. Some disorders can be detected before birth through prenatal diagnosis (screening).

Contents 1 Classification 1.1 Primarily structural 1.2 Other 2 Causes 2.1 Antibiotics 2.2 Petroleum 3 Epidemiology 4 Fossil record 5 See also 6 References 7 External links

Classification

Much of the language used for describing congenital conditions predates genomic mapping, and structural conditions are often considered separately from other congenital conditions. It is now known that many metabolic conditions may have subtle structural expression, and structural conditions often have genetic links. Still, congenital conditions are often classified in a structural basis, organized when possible by primary organ system affected.

Primarily structural

Main article: Congenital abnormality

Several terms are used to describe congenital abnormalities. (Some of these are also used to describe noncongenital conditions, and more than one term may apply in an individual condition.)

• A congenital physical anomaly is an abnormality of the structure of a body part. An anomaly may or may not be perceived as a problem condition. Many, if not most, people have one or more minor physical anomalies if examined carefully. Examples of minor anomalies can include curvature of the 5th finger (clinodactyly), a third nipple, tiny indentations of the skin near the ears (preauricular pits), shortness of the 4th metacarpal or metatarsal bones, or dimples over the lower spine (sacral dimples). Some minor anomalies may be clues to more significant internal abnormalities.
• Birth defect is a widely used term for a congenital malformation, i.e. a congenital, physical anomaly which is recognizable at birth, and which is significant enough to be considered a problem. According to the CDC, most birth defects are believed to be caused by a complex mix of factors including genetics, environment, and behaviors,^[1] though many birth defects have no known cause. An example of a birth defect is cleft palate.
• A congenital malformation is a congenital physical anomaly that is deleterious, i.e. a structural defect perceived as a problem. A typical combination of malformations affecting more than one body part is referred to as a malformation syndrome.
• Some conditions are due to abnormal tissue development:
  • A malformation is associated with a disorder of tissue development.^[4] Malformations often occur in the first trimester.
  • A dysplasia is a disorder at the organ level that is due to problems with tissue development.^[4]
• It is also possible for conditions to arise after tissue is formed:
  • A deformation is a condition arising from mechanical stress to normal tissue.^[4] Deformations often occur in the second or third semester, and can be due to oligohydramnios.
• A disruption involves breakdown of normal tissues.^[4]
• When multiple effects occur in a specified order, it is known as a sequence. When the order is not known, it is a syndrome.

Other

• Genetic disorders or diseases are all congenital, though they may not be expressed or recognized until later in life. Genetic diseases may be divided into single-gene defects, multiple-gene disorders, or chromosomal defects. Single-gene defects may arise from abnormalities of both copies of an autosomal gene (a recessive disorder) or of only one of the two copies (a dominant disorder). Some conditions result from deletions or abnormalities of a few genes located contiguously on a chromosome. Chromosomal disorders involve the loss or duplication of larger portions of a chromosome (or an entire chromosome) containing hundreds of genes. Large chromosomal abnormalities always produce effects on many different body parts and organ systems.
• A congenital metabolic disease is also referred to as an inborn error of metabolism. Most of these are single gene defects, usually heritable. Many affect the structure of body parts but some simply affect the function.
• Other well defined genetic conditions may affect the production of hormones, receptors, structural proteins, and ion channels.

Causes

This section requires expansion.

Antibiotics

Use of antibiotics around the time of conception, particularly sulfonamides and nitrofurantoin are associated with major birth defects. Whether or not this association is causal has not been determined.^[5]

Petroleum

Petroleum oil and distillates cause birth defects.^[6]

Epidemiology

Disability-adjusted life year for congenital anomalies per 100,000 inhabitants in 2004.^[7]

  no data

  less than 160

  160-240

  240-320

  320-400

  400-480

  480-560

  560-640

  640-720

  720-800

  800-900

  900-950

  more than 950

Cell division errors can be due to a lack of nutrients or availability of atomic building blocks, or the presence of toxins that impede normal growth. Division errors which occur very early in the development of a multicellular organism can result in large scale structural and functional differences in the organism's final shape. For example it is now understood that a lack of folic acid in the diet of a mother can cause cellular neural tube deformities that result in Spina Bifida.

External physical shocks or constrainment due to growth in a restricted space, may result in unintended deformation or separation of cellular structures resulting in an abnormal final shape or damaged structures unable to function as expected.

For multicellular organisms that develop in a womb, the physical interference or presence of other similarly developing organisms such as twins can result in the two cellular masses being integrated into a larger whole, with the combined cells attempting to continue to develop in a matter that satisfies the intended growth patterns of both cell masses. The two cellular masses can compete with each other, and may either duplicate or merge various structures. This results in conditions such as conjoined twins, and the resulting merged organism may die at birth when it must leave the life-sustaining environment of the womb and must attempt to sustain its biological processes independently.

Fossil record

Main article: Paleopathology

Evidence for congenital deformities found in the fossil record is studied by paleopathologists, specialists in ancient disease and injury. Fossils bearing evidence of congenital deformity are scientifically significant because they can help scientists infer the evolutionary history of life's developmental processes. For instance, because a Tyrannosaurus rex specimen has been discovered with a block vertebra, it means that vertebrae have been developing the same basic way since at least the most recent common ancestor of dinosaurs and mammals. Other notable fossil deformities include a hatchling specimen of the bird-like dinosaur, Troodon, the tip of whose jaw was twisted.^[8] Another notably deformed fossil was a specimen of the choristodere Hyphalosaurus, which had two heads- the oldest known example of polycephaly.^[9]

See also

• Congenital abnormality
• Malformative syndrome
• ICD-10 Chapter Q: Congenital malformations, deformations and chromosomal abnormalities
• List of congenital disorders
• List of ICD-9 codes 740-759: Congenital anomalies
• March of Dimes
• Mitochondrial disease

References

1. ^ ^{a b} Birth Defects Research. Centers for Disease Control and Prevention.
2. ^ Gregory W. Rutecki (2010). "Pre-Prenatal Care: A Primary Care Primer on the Future". ConsultantLive.com. http://www.consultantlive.com/display/article/10162/1532211. Retrieved 25 September 2010. 
3. ^ Webster's Dictionary. http://www.merriam-webster.com/dictionary/congenital. 
4. ^ ^{a b c d} Graham, John Whichello (2007). Smith's Recognizable Patterns of Human Deformation, 3rd Edition. Philadelphia: Saunders. pp. 3. ISBN 0-7216-1489-2. 
5. ^ Crider KS, Cleves MA, Reefhuis J, Berry RJ, Hobbs CA, Hu DJ (November 2009). "Antibacterial medication use during pregnancy and risk of birth defects: National Birth Defects Prevention Study". Arch Pediatr Adolesc Med 163 (11): 978–85. doi:10.1001/archpediatrics.2009.188. PMID 19884587. 
6. ^ "Pennsylvania, New Jersey – Philadelphia Toxic Tort / Chemical Injury Attorneys". www.lockslaw.com. http://www.lockslaw.com/html/petroleum.html. Retrieved 2010-05-04. 
7. ^ "WHO Disease and injury country estimates". World Health Organization. 2009. http://www.who.int/healthinfo/global_burden_disease/estimates_country/en/index.html. Retrieved Nov. 11, 2009. 
8. ^ Molnar, R. E., 2001, Theropod paleopathology: a literature survey: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, p. 337-363.
9. ^ Ji Q., Wu, X.-C. and Cheng, Y.-N. (2010). "Cretaceous choristoderan reptiles gave birth to live young." Naturwissenschaften, 97(4): 423-428. doi:10.1007/s00114-010-0654-2

External links

• CDC’s National Center on Birth Defects and Developmental Disabilities
• Birth Defects- Types and Causes

v d e Pathology: Medical conditions and ICD code (Disease / Disorder / Illness, Syndrome / Sequence, Symptom / Sign, Injury, etc.) (A/B, 001–139) Infectious disease/Infection: Bacterial disease (G+, G-) · Virus disease · Parasitic disease (Protozoan infection, Helminthiasis, Ectoparasitic infestation) · Mycosis · Zoonosis (C/D, 140–239 & 279–289) Cancer (C00–D48, 140–239) Tumor Myeloid hematologic (D50–D77, 280–289) Anemia · Coagulopathy Lymphoid immune (D80–D89, 279) Immunodeficiency · Immunoproliferative disorder · Hypersensitivity (E, 240–278) Endocrine disease · Nutrition disorder · Inborn error of metabolism (F, 290–319) Mental disorder (G, 320–359) Nervous system disease (CNS, PNS) · Neuromuscular disease (H, 360–389) Eye disease · Ear disease (I, 390–459) Cardiovascular disease (Heart disease, Vascular disease) (J, 460–519) Respiratory disease (Obstructive lung disease, Restrictive lung disease, Pneumonia) (K, 520–579) Stomatognathic disease (Tooth disease) · Digestive disease (Esophageal, Stomach, Enteropathy, Liver, Pancreatic) (L, 680–709) Skin disease · skin appendages (Nail disease, Hair disease, Sweat gland disease) (M, 710–739) Musculoskeletal disorders: Myopathy · Arthropathy · Osteochondropathy (Osteopathy, Chondropathy) (N, 580–629) Urologic disease (Nephropathy, Urinary bladder disease) · Male genital disease · Breast disease · Female genital disease (O, 630–679) Complications of pregnancy · Obstetric labor complication · Puerperal disorder (P, 760–779) Fetal disease (Q, 740–759) Congenital disorder (Congenital abnormality) (R, 780–799) Syndromes · Medical signs (Eponymous) (S/T, 800–999) Bone fracture · Joint dislocation · Sprain · Strain · Subluxation · Head injury · Chest trauma · Poisoning

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