Most infants infected with HIV through mother-to-child transmission (MTCT, or perinatal transmission) become infected during labor and delivery. The purpose of this study is to test the safety and tolerability of a single dose of tenofovir disoproxil fumarate (TDF) or emtricitabine/TDF (FTC/TDF) given at the time of labor to HIV infected pregnant women and to their newborn infants.