A Mixed Methods Exploration of Prenatal Polysubstance Use - PROJECT SUMMARY/ABSTRACT Substance use during pregnancy can lead to negative maternal and infant outcomes. Polysubstance use in pregnancy is particularly salient considering that the increase in number of substances used carries the compounded risk for both the mother and fetus of each individual substance, as well as potential negative interactions between substances. Prenatal polysubstance use in the U.S. is increasing, and understanding of the behavioral patterns of such use is critical. Current research supports the relationship between prenatal individual substance use and biopsychosocial factors. However, there is limited research examining relationships between social determinants of health (SDoH) and prenatal polysubstance use. This information is important given that exposure to adverse SDoH factors can increase the level of stress experienced by individuals, which subsequently increases risk for experiencing substance use issues, posing an equity issue for such vulnerable populations. The overall objective of this research will be to better understand prenatal polysubstance use in the U.S. by approaching the characterization in two ways (qualitative and quantitative) through a socioecological lens, and triangulating findings. Specifically, this project will: 1) conduct qualitative interviews among pregnant or postpartum people engaging in prenatal polysubstance use to understand the biopsychosocial context of such behaviors, and 2) utilize an existing national pregnancy survey (CDC PRAMS) to quantitatively establish psychosocial outcomes of polysubstance use behaviors among pregnant people, which will be the first to incorporate SDoH into such a model. First, 20 qualitative interviews will be conducted with pregnant or postpartum people engaging in polysubstance use to understand the experiences and environment that surrounds such use. Second, the findings of these interviews will inform an exploration of present-day patterns of pregnant polysubstance use using CDC PRAMS data. These patterns will be characterized using latent class analysis (LCA) to define unique groups of polysubstance use, with the five SDoH health domains as external validators: economic stability, education access and quality, healthcare access and quality, neighborhood and built environment, and social and community context. The strength of this approach lies in the specificity of quantitative analyses, while being complemented by the depth of lived experiences. This research will be the first to our knowledge to determine the relationship quantitatively and qualitatively between SDoH and prenatal polysubstance use in the U.S. This dissertation project will lay the foundation for future work to determine factors we may be able to target for prevention, treatment, and initiatives to decrease the burden of prenatal polysubstance use.