The effects of Medicaid expansions on Native American (NA) birth outcomes are understudied. Since the early 1980's, there have been two periods of notable expansions in Medicaid eligibility criteria that have been adopted by different states at varying times. In the late 1980s and 1990s, states increased Medicaid income thresholds for pregnant women to varying degrees. In 2014 and later, under the Affordable Care Act, many states expanded Medicaid thresholds for low- income adults. While the first period primarily may have increased pregnant women's access to prenatal care, the later period of expansion potentially has further increased women's access to pre-conception care. NA mothers have elevated rates of low birth weight (<2,500 gm), preterm birth (gestation < 37 weeks), and macrosomia (birth weight > 4,500 gm). These disparities, reflecting elevated rates of poverty and chronic disease in NA communities, are rooted in systems of structural racism and historical trauma. Medicaid is an important source of health insurance for NAs, covering 67% of NA births as of 2018. As such, Medicaid expansions have the potential to substantially improve NA maternal and infant health. However, NA women also may face barriers to accessing care and to having that care translate into improved infant health. In particular, heterogeneity in communities' economic and health care resources, as well as proximity to Indian Health Services, may influence how Medicaid impacts NA birth outcomes. The proposed work draws on national-level microdata from U.S. birth certificates and employs multiple measures of birth outcomes (i.e., birthweight, gestational age, fetal growth rates, and APGAR scores), capturing distinct aspects of fetal development and infant health. The American Community Survey is also used to assess trends in insurance coverage. Identifying plausibly exogenous Medicaid expansions in two periods (1989-1997 and 2010-2018), this project aims to: (1) Estimate the effects of Medicaid expansions on birth outcomes among NA mothers, testing whether effects differ for NA mothers relative to mothers from other racial- ethnic groups; (2) Evaluate whether Medicaid has differing impacts on infant health depending on the attributes of the counties in which mothers reside; and; (3) Estimate the effects of Medicaid on plausible mechanisms linked to birth outcomes, specifically, insurance coverage, prenatal care, smoking and weight gain during pregnancy, and maternal health risk factors during pregnancy. As health at birth shapes wellbeing and economic outcomes across the life course, a deeper understanding of how Medicaid influences NA maternal and infant health is likely critical to longer-term goals of improving health and economic conditions for future generations of NAs.