Mothers will be followed for 12 weeks postpartum or for 2 years after giving birth if viral resistance to TDF or FTC/TDF is demonstrated at Weeks 1, 6, or 12. In addition to the PK studies, blood collection will occur around the time of delivery, at screening, study entry, at delivery, and after delivery at various times up to Week 12. Physical exams will be done at screening, study entry, at delivery, and after delivery at various times up to Week 8. Infants will be followed until age 2. Blood will be collected and physical exams will be done at birth and at various times up to Week 96. Mothers are encouraged to coenroll in PACTG P1025, Pharmacokinetic Study of Anti-HIV Drugs During Pregnancy.