Studies of antidepressant safety in pregnancy typically do not address complex patterns of use throughout pregnancy. We performed longitudinal trajectory modeling to describe patterns of antidepressant use in the first 32 weeks of pregnancy, and test whether these trajectories are associated with a reduction in birth weight or gestational age at delivery. Our study included 166 pregnant women with deliveries between 2011 and 2015 who were prescribed an antidepressant between 91 days prior to last menstrual period and 32 weeks of gestation. From electronic medical records, we estimated average daily dose and cumulative dose per week for the first 32 weeks of gestation and for the first 13 weeks postnatal. We clustered women with similar utilization patterns using k-means longitudinal modeling and assessed the associations between trajectory group and birth weight and gestational age at delivery. We identified four cumulative dose trajectory groups and three average daily dose trajectory groups in each period. Relative to the lowest trajectory group, the highest trajectory group during pregnancy was associated with reduced birth weight in multivariable analysis (average daily highest trajectory vs. lowest trajectory beta - 314.1 g, 95% CI - 613.7, - 15.5) adjusted for depression severity score, maternal age, race, and pregnancy smoking. Trajectory groups were not associated with gestational age at delivery. The highest trajectory group of antidepressant use in pregnancy was associated with a modest reduction in birth weight but not with gestational age at delivery. Longitudinal trajectories allow for a dynamic visualization and quantification of medication use among pregnant women.