Objectives: This study examined the effects of prenatal participation in the NYS WIC Program on birth weight through enhanced control of selection bias and gestational age bias. Program effects were assessed separately for White, Black, and Hispanic women and subpopulations defined by values of Kotelchuck index of adequacy of prenatal care utilization.
Methods: 1995 New York State Vital Statistics records were linked to WIC certifications, administrative and check redemption files, and to the 1990 federal census of NY county level data. The final data set contained 77,601 records. Birth weight among WIC participants who enrolled early and participated longer were compared to those who enrolled late and participated a shorter time. Selection bias was addressed using classification tree methods as part of a propensity score analysis. Gestational age bias was addressed by analyzing preterm and full-term pregnancies separately.
Results: Adjusted estimates showed a significant positive effect of longer prenatal WIC participation on birth outcomes for all groups studied. Infants born to WIC participants who enrolled early were heavier than those who enrolled late by, on average, 70 g for full-term and 129 grams for preterm. Black and Hispanic full-term infants experienced larger WIC effects than Whites (79, 75, 43 g, respectively). Looking at full-term pregnancies using Kotelchuck's index indicated that effects of longer prenatal WIC participation were greatest for the inadequate prenatal care group (83 g).
Conclusion: Longer prenatal WIC participation was associated with an increase in birth weight overall and for all groups studied. The effect on birth weight of longer participation in WIC was greatest in Black and Hispanic, inadequate and no prenatal care groups.