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Objective To examine the association between the Special Supplemental Nutrition Program for Women, Infants, and Children (WIC) participation during pregnancy and birth weight by race and ethnicity on a national level. Design Retrospective cohort of US natality from 2014 to 2022. Setting All US states and Washington, DC. Participants First-time mothers of a live singleton birth covered by Medicaid with available data on WIC participation and birth weight. Exposure WIC participation during pregnancy (yes/no) Main Outcome Measure Low (< 2,500 g) and very low (< 1,500 g) birth weight. Analysis Multivariable logistic regression Results A total of 3,778,400 mothers covered by Medicaid were included in this analysis, of which 69.6% were WIC participants during pregnancy. The association between WIC participation and birth weight outcomes differed by race and ethnicity. After adjusting, WIC participants during pregnancy were 25%, 20%, 17%, and 10% less likely to give birth to low-weight infants compared with non-WIC participants for Native Hawaiian or Other Pacific Islanders, Black, American Indian or Alaskan Native, and White, respectively. Conclusion and Implications Special Supplemental Nutrition Program for Women, Infants, and Children participation during pregnancy was associated with reduced low birth weight in all races and ethnicities. Improving WIC access and participation during pregnancy may support health equity in birth weight outcomes.