OBJECTIVE: The purpose of this study was to describe antidepressant medication use patterns during pregnancy and pregnancy outcomes. STUDY DESIGN: We evaluated a cohort of 228,876 singleton pregnancies that were covered by Tennessee Medicaid, 1995-2007. RESULTS: Of 23,280 pregnant women with antidepressant prescriptions before pregnancy, 75% of them filled none in the second or third trimesters of pregnancy, and 10.7% of them used antidepressants throughout pregnancy. Filling 1, 2, and ≥3 antidepressant prescriptions during the second trimester was associated with shortened gestational age by 1.7 (95% confidence interval [CI], 1.2-2.3), 3.7 (95% CI, 2.8-4.6), and 4.9 (95% CI, 3.9-5.8) days, when controlled for measured confounders. Third-trimester selective serotonin reuptake inhibitor use was associated with infant convulsions; adjusted odds ratios were 1.4 (95% CI, 0.7-2.8); 2.8 (95% CI, 1.9-5.5); and 4.9 (95% CI, 2.6-9.5) for filling 1, 2, and ≥3 prescriptions, respectively. CONCLUSION: Most women discontinue antidepressant medications before or during the first trimester of pregnancy. Second-trimester antidepressant use is associated with preterm birth, and third-trimester selective serotonin reuptake inhibitor use is associated with infant convulsions. © 2012 Mosby, Inc. All rights reserved.