Participants will come to the NIH Clinical Center once every 6 to 12 weeks until around their 34th week (8 months) of pregnancy and then again at least 1 month after the birth of the baby to have blood drawn. A catheter (thin plastic tube) will be placed in a vein to avoid multiple needle sticks for blood sampling during the day. The first sample will be collected before the patient takes the morning doses of anti-HIV medicines and additional samples will be drawn at 1, 2, 4, 8 and 12 hours after taking the medication. A urine sample will also be collected at each visit.