Birth defects, also known as congenital malformations, are structural or metabolic abnormalities present at birth. While subtle variations from the normal, of no clinical interest, occur in about half of all individuals in the United States, significant congenital defects are found in about 3% of live births. Fortunately, only about half of these require medical attention.

Birth defects may result from genetic causes or environmental insult. Defective genes are not easily repaired and thus are perhaps less interesting than teratogenic substances to environmentalists. It is theoretically possible to limit exposure to teratogens by elimination of the agent in the environment or by modification of behavior to prevent contact. It should be noted, however, that the causes of more than half of congenital malformations remain unknown.

Birth defects of genetic origin may be due to aberrant chromosome number or structure, or to a single gene defect. Normal humans...