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
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.
HARRY W. SCHROEDER, JR.
(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)
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