Participants in this double blind study will initially report to the study site two separate times while they are pregnant. At the first visit, which will last approximately 3 hours, medical and psychiatric histories will be taken. Blood and urine samples will also be collected at this time. At the second visit, which will last approximately 1 hour, participants will be randomly assigned to receive either sertraline or placebo for the year following childbirth. All participants will then either attend 10 clinic visits, each lasting 1 hour, or receive 17 phone calls over the course of the first 28 weeks postpartum. Mood symptoms and day-to-day functioning will be assessed. If the participant's baby is being breastfed, a blood sample will also be taken from the baby at Week 4 postpartum. At Week 12 postpartum, the baby will wear an actigraph to measure his or her activity for one week. At Week 24 postpartum, participants who are not depressed will be randomly assigned to either continue on sertraline or taper to placebo over 4 weeks. Those who were originally assigned to receive placebo will continue taking placebo for the remainder of the study. Starting at Week 29 postpartum, there will be approximately one clinic visit per month for the remainder of the year. Participants' functioning, ability to interact with their children, and symptoms of depression will be assessed at these visits.