PROJECT SUMMARY/ABSTRACT Gestational diabetes mellitus (GDM) is characterized by maternal hyperglycemia leading to fetal overgrowth and associated complications: large for gestational age (LGA), neonatal hypoglycemia, shoulder dystocia, birth injury, and cesarean birth. While insulin has been the traditional pharmacologic intervention to maintain euglycemia among women with GDM, there are numerous barriers to insulin use. Insulin therapy often requires referral to specialized providers, training on administration, and multiple daily injections – all factors that may deter effective utilization, particularly in women from racial/ethnic minorities or living in rural areas. Disparities in diagnosis and treatment of GDM may have large public health implications; non-white women are more than twice as likely to be diagnosed with GDM compared to white women, and black women have higher risks for GDM-associated pregnancy and neonatal complications compared to white women. Newer oral hypoglycemic agents, such as metformin, may be safe and effective alternatives to insulin that are more widely accessible and acceptable in minority or resource-limited populations. The American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine continue to disagree about the optimal pharmacological treatment for GDM. Furthermore, to date, few studies have examined how patient sociodemographic factors - especially race/ethnicity, rural residence, and socioeconomic status - impact the relationship between hypoglycemic medication selection and perinatal outcomes. Although the management of GDM ideally would be informed by properly powered randomized clinical trials, the medications of interest are already on the market and new trials are unlikely. Our multidisciplinary team of experts in maternal-fetal medicine, health policy, pharmacoepidemiology, biostatistics, and computer sciences have demonstrated successful collaboration to investigate medication use during pregnancy and the postpartum period. We propose a carefully designed observational cohort study using a comprehensive research platform of linked records including TN Medicaid claims, birth certificates and a registry of all hospital-based encounters in TN, which will focus on relevant clinical outcomes and incorporate the practical complexities of real-world settings. Our proposal aims to test the hypotheses that metformin use in women with GDM is associated with a lower risk of adverse neonatal (Aim 1) and maternal (Aim 2) outcomes compared to glyburide or insulin, especially among women from sociodemographic minority groups. Key neonatal outcomes include large for gestational age, neonatal hypoglycemia, and NICU admission and key maternal outcomes include cesarean birth, new hypertensive disorders of pregnancy, and third- and fourth-degree perineal lacerations.