The expected duration of participation for individually enrolled participants will depend on how early they enroll prior to the date of cesarean section, but may range from approximately 3 to 6 weeks. An initial screening/enrollment visit will occur approximately 1 to 4 weeks prior to the participant's scheduled cesarean section, but no more than 4 weeks before the delivery date. A targeted physical exam, medical and medication history assessment, blood and urine collection, and a pelvic exam will occur at screening/enrollment. Tenofovir gel will be administered vaginally approximately 2 hours before the expected time of cesarean section. On the day of gel administration and cesarean delivery, a targeted physical exam, medical and medication history assessment, a pelvic exam, maternal blood tenofovir level measurement, and collection of placental and endometrial tissues, cord blood, and amniotic fluid will occur.