Background: Few studies investigated longitudinal antidepressant exposure during pregnancy and included dosage in the assessment. Methods: We conducted a nationwide, registry-linkage study in Norway using data on antidepressant prescription fills in pregnancies lasting ≥32 weeks in women with a delivery between 2009 and 2018 who had a depression/anxiety diagnosis and antidepressant fills prior to pregnancy. Information on antidepressant exposure by week (measured by filled prescriptions) and prescribed average daily dose was used in longitudinal k-means trajectory modelling for a 108-week time window from six months prior to pregnancy to one year after delivery. Factors associated with trajectory group membership were examined using multinomial logistic regression models. Results: We included 8,460 pregnancies in 8,092 women. Four antidepressant fill trajectories were identified based on filled antidepressant prescriptions: two distinct discontinuing patterns, one at around the start of pregnancy (30.4%) and one around the end of pregnancy (33.8%); one continuing pattern (20.6%); and one interrupting pattern (15.2%). Using average usual daily dose, we identified low dose discontinuing (60.3%), medium dose reducing (20.6%) and high dose continuing (15.2%) patterns. The multinomial logistic regressions showed that the fill trajectory group membership was strongly associated with: antidepressant type and dose prior to pregnancy and co-medication prior to pregnancy, maternal age, marital status, parity, previous pregnancy loss, and pregnancy planning. Conclusion: Longitudinal trajectory modelling revealed distinct antidepressant fill and dosage patterns in the period around pregnancy. Knowledge about factors associated with utilization trajectories might be useful for health-care personnel counselling women about antidepressant use in pregnancy.