Rates of MTCT of HIV have dramatically decreased in resource-rich countries since the introduction of antiretroviral (ARV) prophylaxis; increased prenatal care, HIV testing, and counseling; elective cesarean delivery; and avoidance of breastfeeding. In resource-limited countries, however, MTCT of HIV continues to be a widespread problem. In these parts of the world, ARV prophylaxis is too expensive and too difficult to adequately administer; mothers often do not receive proper prenatal care; cesarean delivery may pose risks to the mother and and her infant; and due to the lack of safe, affordable, and socially acceptable alternatives, HIV infected mothers breastfeed their infants. The purpose of this study is to evaluate the safety, tolerability, and pharmacokinetics (PK) of TDF in HIV infected pregnant women and their infants.