PROJECT SUMMARY/ABSTRACT Prenatal marijuana use is increasingly recognized as an important public health problem that could have a large scale negative impact on maternal, fetal and neonatal health. We propose a retrospective and prospective longitudinal cohort study of over 400,000 pregnancies screened for prenatal marijuana use based on gold standard self-reported and urine toxicology data from 2009 to 2020, leveraging Kaiser Permanente Northern California’s universal prenatal substance use screening program that is part of standard prenatal care. We will assess associations between maternal marijuana use during pregnancy and key maternal, fetal and neonatal outcomes, controlling for potential confounders (e.g., tobacco use, alcohol and other drug exposure, income) and comparing exposure across trimesters. These analyses also take advantage of a unique natural experiment and will examine whether California state marijuana legalization in 2018 and local regulatory practices (e.g., retailer bans, prominent warnings on marijuana use during pregnancy) are associated with variation in marijuana use during pregnancy. With high generalizability and a sample size unmatched by any other study on this topic, results from each aim have direct implications for the care of pregnant women while providing novel insight to drive future epidemiological and clinical trial-based intervention studies. Aims 1 and 2 will provide generalizable and comprehensive data to understand the health impact of prenatal marijuana use on mothers and their babies. Aim 3 will allow us to identify changing trends in prenatal marijuana use following marijuana legalization and will inform clinicians, researchers, and policy makers of the future scope of the problem. Finally, the study cohort will be a valuable resource for future research on the long-term impact of prenatal marijuana use on maternal and children’s well-being.