We analyze the relationship between prenatal WIC participation and birth outcomes in New York City from 1988 - 2001. The analysis is unique for several reasons. First, we have over 800,000 births to women on Medicaid, the largest sample ever used to analyze prenatal participation in WIC. Second, we focus on measures of fetal growth distinct from preterm birth, since there is little clinical support for a link between nutritional supplementation and premature delivery. Third, we restrict the primary analysis to women on Medicaid who have no previous live births and who initiate prenatal care within the first four months of pregnancy. Our goal is to lessen heterogeneity between WIC and non-WIC participants by limiting the sample to highly motivated women who have no experience with WIC from a previous pregnancy. Fourth, we analyze a large sub-sample of twin deliveries. Multifetal pregnancies increase the risk of anemia and fetal growth retardation and thus may benefit more than singletons from nutritional supplementation. We find no relationship between prenatal WIC participation and measures of fetal growth among singletons. We find a modest pattern of association between WIC and fetal growth among U.S.-born Black twins. Our findings suggest that prenatal participation in WIC has had a minimal effect on adverse birth outcomes in New York City.