To prevent mother-to-child transmission (MTCT) of HIV in resource-limited countries, a simple yet effective treatment plan is needed. Tenofovir disoproxil fumarate (TDF) is an anti-HIV drug approved for use in the United States for the treatment of HIV infected adults. The purpose of this study is to determine the safety, tolerability, and blood levels of TDF in HIV infected pregnant women and their babies. The study will be conducted at sites in Malawi and Brazil.