Some viruses, such as rubella and human cytomegalovirus, are known to cross the placental barrier and infect the fetus. In other cases of maternal viral infections, such as herpes simplex, evidence for transplacental passage is less convincing and fetal damage or neonatal disease may be coincidental or associated with perinatal infection. Certain cases of fetal or neonatal disease following maternal viral infections may be associated with disease in the mother which affects her metabolic processes or the placenta in such a way as to interfere with development of the fetus and infant. The possible effects of transplacental viral infections are several. Fetal loss may occur by means of abortion or stillbirth. There may be infection of the fetus, with clinical manifestations such as rash, or without clinical manifestations. The infant may be born with congenital defects, including such deformities as cataracts, cardiac anomalies, mental retardation or cerebral palsy. Although a number of maternal viral diseases have been etiologically incriminated in congenital defects, only two-rubella and cytomegalovirus infection-are definitely proved to be associated with anomalies or mental retardation in infants.