Project summary/abstract Cannabis use during pregnancy is a growing public health concern in the United States as more states are legalizing medical and recreational use. Its prenatal use has significant implications for the health of the mother and her developing child. Tetrahydrocannabinol (THC), the main psychoactive constituent of cannabis, crosses the placenta, with greater transfer during the early stages of pregnancy. Thus, it is important to understand the prevalence and effects of prenatal cannabis using objective methods, since the prenatal period is a unique window of opportunity to prevent adverse neonatal outcomes. The present study will be based on the NIH-funded MARCH (Michigan Archive for Research in Child Health) open cohort that represents a stratified random sample of births in the lower peninsula of Michigan. This ongoing cohort, initiated in 2017, collects questionnaires, medical records and biological samples beginning with the first prenatal visit until later childhood, accruing a wealth of data on prenatal exposures and outcomes. Given this rich database, we propose to examine the following specific aims: Aim 1: Assess agreement between self-reported cannabis use and urine toxicology measures and determine correlates of non-disclosure; Aim 2: Assess the prevalence and associated characteristics (socioeconomic status, demographic factors, psychosocial wellbeing, and concurrent use of other substances) of cannabis use during pregnancy; Aim 3: Assess the effect of prenatal cannabis use on key neonatal outcomes; and Aim 4: Explore the effect of prenatal cannabis use on important obstetric/maternal outcomes.