HIV is transmitted from an infected woman to her fetus or newborn during pregnancy or delivery or through breast-feeding. HIV detected in fetal tissues and HIV isolated in cord blood provide suggestive evidence that transmission can occur in utero, but recent studies suggest much if not most transmission occurs during the intrapartum period. HIV can be detected at birth by culture or polymerase chain reaction (PCR) in only 30 to 50% of infants ultimately found to be infected. There are several reports of mothers who were infected through postpartum transfusions and subsequently transmitted HIV to their infants through breast-feeding.

For that reason, the USPHS strongly recommends that HIV-positive mothers avoid breast-feeding in the United States, where nutritionally adequate and safe substitutes are available.

The rate of perinatal transmission in published studies varies from 13 to >40%, with higher rates associated with advanced maternal HIV disease or increased viral load and reduced CD4 count, higher rates of breast-feeding, and the presence of chorioamnionitis with more than 4 hours of rupture of membranes.

In an important randomized, placebo-controlled trial (ACTG 076), zidovudine administered to asymptomatic HIV-infected pregnant women with CD4 counts >200 per cubic millimeter reduced perinatal transmission by two thirds. The regimen was initiated after the thirteenth week of gestation and accompanied by intravenous zidovudine during parturition and 6 weeks of therapy to the newborn. Based on these data, the USPHS issued guidelines on use of zidovudine during pregnancy to reduce perinatal transmission of HIV infection. The implementation of such recommendations has lead to a 43% reduction in perinatally acquired HIV infection between 1992 and 1996 in the United States. In 1997, less than 500 cases of pediatric AIDS were reported in the United States.