We develop county-level measures of structural and institutional barriers to care, and test associations between these barriers and birth outcomes for US-born Black and White mothers using national birth records for 2014-2017. Results indicate elevated odds of greater preterm birth severity for Black mothers in counties with higher uninsurance rates among Black adults, fewer Black physicians per Black residents, and fewer publicly-funded contraceptive services. Most structural barriers were not associated with small-for-gestational-age birth, and barriers defined for Black residents were not associated with birth outcomes for White mothers, with the exception of Black uninsurance rate. Structural determinants of care may influence preterm birth risk for Black Americans.