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Importance: Cannabis use among pregnant individuals has increased. Depression and stress are frequently reported motives for cannabis use that may prolong using cannabis during pregnancy., Objective: To examine associations between changes in depression, stress, and self-reported prenatal cannabis use (PCU), to examine motives for PCU, and to examine whether trajectories of depression and stress vary across individuals who report using cannabis to cope with mental health symptoms and/or stress, those who use cannabis for other reasons, and those who do not report PCU., Design, Setting, and Participants: This cohort study recruited pregnant individuals at an obstetric clinic at an academic hospital between July 2019 and January 2024 and followed them during pregnancy. Pregnant individuals with a history of lifetime cannabis use were included. Individuals reporting heavy episodic alcohol use or with other illicit drug use were excluded., Exposure: Self-reported PCU., Main Outcomes and Measures: The primary outcomes were self-reported depression (Edinburgh Postnatal Depression Scale), stress (Cohen Perceived Stress Scale), and cannabis use at each trimester during pregnancy, as well as motives for cannabis use during the first trimester (T1). Stability and changes in depression and stress scores and categorical self-reported prenatal cannabis use from T1 to the third trimester (T3) were estimated using individual linear growth curve models., Results: In this sample of 504 patients (all identified as women; median [IQR] age, 26 [18-40] years), 236 individuals (46.8%) reported PCU after pregnancy knowledge. Depression, stress, and PCU decreased from T1 to T3 (all slope estimates less than -0.29; SEs, 0.23-0.7; all P < .001). There were positive associations between depression and PCU at T1 (r = 0.17; P = .004) and in their rate of change (r = 0.18; P = .01). Only T1 stress and PCU were correlated (r = 0.14; P = .004). Participants reporting PCU for mental health reasons (137 participants [58.1%]) had the highest depression scores at each trimester; however, their rate of change in depression was statistically equivalent to those who did not use cannabis., Conclusions and Relevance: In this cohort study of PCU, participants who used cannabis did not experience a more significant decline in stress or depression symptoms compared with those who did not use cannabis. Individuals who used cannabis for mental health reasons did not hasten a decrease in their symptoms. Health care professionals are encouraged to enhance prenatal individuals' access to empirically supported treatments for depression and stress.