PROJECT SUMMARY Although lifestyle factors have been investigated for their potential to modify risk of adverse pregnancy outcomes, initiating interventions during pregnancy may not have the greatest impact. Therefore, it is crucial to focus on preconception interventions to support healthy placental development and reduce the risk of metabolic conditions. However, there is a lack of data regarding the optimal timing and strategies for preconception health to improve pregnancy outcomes. Physical activity, diet, and sleep play pivotal roles in preconception care as they influence metabolic health and body weight regulation, which can impact the subsequent risk of pregnancy complications. Existing preconception care often emphasizes weight loss, but there is limited evidence on the most effective amounts and strategies for improving later pregnancy health. Moreover, using body mass index as the primary measure for obesity classification may misidentify individuals who would benefit the most from weight loss, as it fails to distinguish between lean mass, fat mass, and fat distribution. Methodological challenges in studying preconception exposures have hindered the establishment of a robust evidence base for optimizing pregnancy health. To address these gaps, we propose an innovative approach utilizing the linkage between the National Health and Nutrition Survey (NHANES) and Medicaid data. With this data examine pregnancy outcomes among low-income reproductive-aged women who become pregnant and utilize Medicaid services following their NHANES assessment. Medicaid covers a substantial portion of U.S. births, making studies of Medicaid-covered pregnancies highly generalizable. Leveraging the NHANES-Medicaid data as a "preconception cohort," we will investigate the impact of preconception exposures on pregnancy outcomes. Employing advanced epidemiological methods to minimize selection bias and improve generalizability, we will apply a target trial framework to assess preconception strategies for optimizing pregnancy outcomes. The comprehensive phenotyping data available in NHANES will enable robust assessment of confounding variables, reducing the impact of unmeasured confounders and mimicking randomized trials of lifestyle factors. Using NHANES-assessed exposures, we will evaluate the influence of preconception dietary patterns, physical activity, sleep, body composition, and weight loss attempts and strategies on adverse pregnancy outcomes. This analysis will provide the most extensive and generalizable evidence to date on preconception health and its impact on adverse pregnancy outcomes, ultimately informing the development of evidence-based preconception care guidelines.