SUMMARY Alcohol is the most commonly used licit substance in pregnancy, and cannabis is the most commonly used illicit substance in pregnancy. Approximately half of pregnant people who use cannabis also use alcohol, and one third of pregnant people who used alcohol also used cannabis. Both substances are associated with adverse outcomes among offspring, including preterm birth, small for gestational age offspring, and adverse neurodevelopmental outcomes, including fetal alcohol spectrum disorders in alcohol-exposed offspring. There is accumulating evidence that prenatal co-exposure to cannabis and alcohol confers greater risks to the developing fetus than either exposure alone, although data are primarily from animal models. Further, there is no evidence from pregnant populations of whether cannabis is used simultaneously with alcohol, and whether cannabis substitutes or complements alcohol use over the highly dynamic time of pregnancy. Of additional concern, there is accumulating evidence that paternal preconceptional exposure to cannabis and alcohol may directly and indirectly increase the risk of adverse offspring outcomes. The purpose of this developmental proposal is to gather information about alcohol and cannabis use before, during and after pregnancy, for both the pregnant individual and the biologic father, to support the feasibly of a larger cohort in the future. This proposal will elucidate distinct patterns of alcohol and cannabis use and co-use that are necessary to power future studies, and the feasibility of enrolling and interviewing biologic fathers about their own use. Our team includes a perinatal epidemiologist with research expertise in pregnancy cohort studies and prenatal alcohol and cannabis use, a Maternal-Fetal-Medicine specialist with research expertise in alcohol, cannabis and paternal exposures, and a psychiatrist who specializes in addiction during pregnancy. From San Diego County, we will recruit 100 pregnant individuals who used cannabis and alcohol in the three months prior to pregnancy, and their male partners. We will collect detailed information on alcohol and cannabis use and co- use over the course of the pregnancy, using methods shown to be reliable in other pregnancy cohort studies. The aims are to 1) Characterize the patterns of maternal alcohol and cannabis use before, during, and after pregnancy, 2) Characterize the patterns of paternal alcohol and cannabis use before, during, and after pregnancy, and 3). Model individual and joint trajectories of exposure for pregnant individuals and their partners. Success in this project will reveal the patterns of alcohol and cannabis use before, during and after pregnancy, by the pregnant person and the biologic father. These data are critical to inform future studies of the direct and indirect effects of parental use of alcohol and cannabis, and to inform intervention efforts and guide counseling.