Women who are 20 0/7 weeks to 37 6/7 weeks pregnant will be enrolled in this study and will remain in the study for up to 12 weeks after delivery. Postpartum women will be enrolled at 2 to 12 weeks after delivery and followed until 6 to 7 weeks after starting contraceptives. Infants will be followed for 16 to 24 weeks of life. At all study visits, participants will undergo a medical history, a physical exam, and blood collection. At some visits, women in some arms will undergo a vaginal swab. Blood collection from the mother and the detached umbilical cord will occur during delivery. Intensive PK sampling will be performed at study visits during the second and third trimester of pregnancy and/or postpartum, depending on the study arm. Additional study visits may occur depending on the ARV drug regimen prescribed.