Cohort 1 is now closed. Each participant in Cohort 1 received a single 600 mg oral dose of TDF at the start of active labor or 4 hours prior to C-section, with concurrent administration of standard intravenous zidovudine (ZDV) prophylaxis and/or other antiretrovirals prescribed by her physician. The infants from Cohort 1 received only the standard 6 weeks of oral ZDV prophylaxis postpartum. PK blood samples were taken from mothers at predose and 1, 2, 4, 8, 12, and 24 hours postdose and at the time of delivery; PK blood samples were taken from infants at 12, 24, and 36 hours after birth.