PROJECT ABSTRACT Residential segregation is the geographic separation of low income and historically marginalized individuals into distinct environments. For many U.S. residents, residential segregation serves as both a reflection and reinforcement of structural racism. Although residential segregation has been identified as a root cause of disparities in the U.S., long-term consequences of residential segregation remain largely understudied, including environmental and reproductive consequences such as the racist siting of hazardous industries (e.g., oil and gas development [OGD] sites) and fertility (fecundability, the per-cycle probability of pregnancy). OGD activities are rapidly expanding across North America and projected to continue through 2032, many of which are disproportionately sited in marginalized or racially devalued communities. OGD activity also poses a significant threat to reproductive health, which may be amplified in segregated neighborhoods (an environmental framework known as “fossil fuel racism”). Epidemiologic research also indicates residential segregation influences health outcomes across the reproductive continuum, which may be modified by environmental hazards given racialized economic patterns of (dis)investment, however its impact on fecundability is not known. In this application, we evaluate environmental and reproductive consequences of residential segregation during the preconception period. We will use prospective cohort data from Pregnancy Study Online (PRESTO), a North American preconception cohort study, which is the largest and most diverse preconception cohort in existence (n>15,000 U.S. participants). PRESTO has remarkable geographic, racial, and socioeconomic diversity, with participants enrolled across all 50 U.S. states, and >20% of participants identify as Black, Indigenous, or People of Color (BIPOC). Reproductive-aged couples are recruited and followed for up to 12 months or until pregnancy using internet-based methods, and a wealth of data are collected that makes PRESTO uniquely suited to evaluate complex influences of residential segregation. We will use these data to address the following aims: 1) characterize exposure to residential segregation, 2) estimate the effect of residential segregation on the prevalence of OGD sites in disadvantaged neighborhoods, and 3) estimate the effect of residential segregation on fecundability, including effect modification by OGD activity. This diversity supplement will also support Ms. Sharonda Lovett’s training to build and strengthen skills to achieve her long-term career goal. Under the mentorship of Dr. Mary Willis (an expert in geospatial exposure assessment, applied data science, and environmental exposures), this diversity supplement will support Ms. Lovett (the candidate), develop expertise in geospatial exposure assessment and environmental epidemiology, both of which are areas she has not had formal training to date. The tailored career development p...