Little is known about antidepressant medication use patterns during pregnancy among periconception (before and immediately following conception) users. Additionally, the associations between these patterns and birth outcomes is unclear, after taking into account underlying depression severity. This study describes patterns of antidepressant use among periconception users and examines associations between usage patterns and birth outcomes. This retrospective cohort study included pregnant Kaiser Permanente Northern California (KPNC) members with a live birth between 2014 and 2017 and an antidepressant medication fill that overlapped the 8th week of pregnancy. Outcomes were preterm birth and neonatal intensive care unit (NICU) admission. Data were extracted from KPNC’s electronic health records. Modified Poisson regression was conducted. Of the 3637 pregnancies meeting inclusion criteria, 33% (n = 1204) continued antidepressant use throughout the pregnancy (refilled throughout pregnancy), 47% (n = 1721) discontinued use (no refills), and 20% (n = 712) stopped and reinitiated use (refill after 30+ day gap in supply). Women who continued use had 1.86 (95% confidence interval (CI) 1.53, 2.27) times the risk of preterm birth and 1.76 (95% CI: 1.42, 2.19) times the risk of NICU admission, compared to women who discontinued use during pregnancy. Similarly, women with continued use had 1.66 (95% CI: 1.27, 2.18) times the risk of preterm birth and 1.85 (95% CI: 1.39, 2.46) times the risk of NICU admission, compared to women who stopped and reinitiated use. This relationship held when examining continuous exposure; the relationship between continuous exposure and preterm delivery was stronger in later trimesters. Periconception antidepressant users who continue use during pregnancy, particularly into the second and third trimesters, may be at higher risk of adverse birth outcomes. This evidence should be considered alongside the risks associated with depression relapse.