This study examines how educational assortative mating (EAM) and marital status jointly shape risks of low birthweight (LBW) and macrosomia using 2024 National Vital Statistics System natality data (N = 2,906,124). Birthweight reflects cumulative social, behavioral, and biological processes during pregnancy. Prior research documents strong educational gradients but typically treats parental education and marital status as independent predictors. Guided by family systems and resource multiplication theories, this study investigates how joint parental educational pairing – and its interaction with marital status – structures risk at both ends of the birthweight distribution. Multinomial logistic regression models estimate relative risks of LBW and macrosomia versus normal birthweight as functions of EAM, marital status, and their interaction with adjustments. Results show that among unmarried women, relative to heterogamy, medium- and high-educated homogamy are associated with lower LBW risk and higher macrosomia risk, whereas low-educated homogamy shows little difference in LBW risk. Among heterogamous couples, marriage is protective against LBW but increases macrosomia risk; within marriage, however, educational homogamy attenuates this risk. Marital advantages are increasingly concentrated among medium- and high-educated homogamous unions and are substantially weaker among low-educated homogamous couples. In sum, marriage amplifies advantages among medium- and high-educated homogamous couples but does not offset disadvantage among low-educated homogamous families, underscoring assortative mating as a mechanism of early-life health inequality. Findings position marriage as a conditional – rather than universal – health-protective institution, highlighting the need for policies that move beyond marriage promotion to address structural educational and economic disadvantages that shape unequal birth outcomes.