Congenital malformations are a common cause of illness, handicapping conditions, and death. Errors of morphogenesis leading to congenital anomalies have many recognized causes, including specific single-gene mutations, chromosome imbalances, and the action of teratogenic agents. Most congenital anomalies, however, are of multifactorial origin, occurring by complex interactions between not yet well understood genetic and environmental factors. Two to six percent of the newborn babies worldwide, i.e., 3 to 9 million infants a year, suffer from major congenital anomalies and genetic diseases. However, a great deal of this misery could be avoided. In particular, preconception care is more important than prenatal care for prevention of congenital anomalies since as many as 30 percent of pregnant women begin traditional prenatal care in the second trimester (>13 weeks of gestation), which is after the period of maximal organogenesis. Women and their partners whose ethnic background, race, or personal or family history places them at increased risk of having a fetus with a genetic disease should receive appropriate counseling. Furthermore, certain laboratory tests of the infection status of couples may be helpful in assessing the risk for and preventing some complications during pregnancy. Preconception evaluation and appropriate counseling permits parents to make informed reproductive decisions and provides reassurance.