ABSTRACT Marijuana use during pregnancy is increasing rapidly in the US. The spreading legalization and growing acceptance, accessibility, and use of marijuana during pregnancy raise important public health concerns for pregnant women as our understanding of the developmental, behavior, and mental health impacts on children exposed to marijuana in utero is alarmingly inadequate. To address this critical research gap, we propose a longitudinal birth cohort study of over 420,000 children born between 2010-2021 at Kaiser Permanente Northern California (KPNC) hospitals. We will leverage data from KPNC's comprehensive electronic health records (EHR) and unique universal prenatal drug use screening program which screens 40,000 women annually via ?gold standard? self-report and urine toxicology tests for marijuana and other substance use. As part of standard care, all children are universally screened for developmental delays at well-child visits and as they grow older universal mental health screening captures depression and substance use disorders. Information on child neurodevelopment, behavior and mental health screening outcomes and diagnoses will be ascertained from KPNC's EHR and through linkage to California's Department of Developmental Services. We will ascertain information on important confounders (e.g., maternal mental health, maternal age, tobacco and other substance use, preterm delivery) from KPNC's EHR. Children born to women with singleton gestations who are screened for prenatal marijuana and other substance use will be included. We will evaluate associations between prenatal marijuana exposure and the risk of neurodevelopmental impairments, behavioral disorders, and mental health outcomes from birth to 13 years of age, after accounting for potential confounders. We will also test for sex differences in these associations. Additional analyses will assess the effects in children whose mothers screened positive for marijuana only during pregnancy (i.e., no other prenatal substance use). The proposed study leverages a diverse population, and with high generalizability, will be the largest to date. Findings will have significant public health and clinical implications and can be used by women to make informed decisions about marijuana use during pregnancy. Additionally, the study cohort will be a valuable resource for future research on the longer-term impact of prenatal marijuana use on children's health.