Objectives Women with longer, healthier pregnancies have more time to enroll in the Special Supplemental Nutrition Program for Women, Infants, and Children (WIC), biasing associations between WIC and birth outcomes. We examined the association between WIC and preterm birth (PTB), low birth weight (LBW), and perinatal death (PND) using a fetuses-at-risk approach to address this bias, termed gestational age bias. Methods We linked California Medi-Cal recipients with a singleton live birth or fetal death from the 2010 Birth Cohort to WIC participant data (n = 236,564). We implemented a fetuses-at-risk approach using survival analysis, which compared, in each week of gestation, women whose pregnancies reached the same length and who had the same opportunity to utilize WIC. In each gestational week, we assessed WIC enrollment and the number of food packages redeemed thus far and computed hazard ratios (HR) using survival models with time-varying exposures and effects. Results Adjusting for maternal socio-demographic and health characteristics, WIC enrollment was associated with a lower risk of PTB from week 29-36 (HR29 = 0.71; HR36 = 0.52); LBW from week 26-40 (HR26 = 0.77; HR40 = 0.64); and PND from week 29-43 (HR29 = 0.78; HR43 = 0.69) (p < 0.05). The number of food packages redeemed was associated with a lower risk of PTB from week 27-36 (HR27 = 0.90; HR36 = 0.84); LBW from week 25-42 (HR25 = 0.93; HR42 = 0.88); and PND from week 27-46 (HR27 = 0.94; HR46 = 0.91) (p < 0.05). Conclusions for Practice To our knowledge this is the first study to examine the association between WIC and birth outcomes using this approach. We found that beginning from about 29 weeks, WIC enrollment was associated with a reduced risk of PTB by 29-48 %, LBW by 23-36 %, and PND by 22-31 %.
Keywords: Birth outcomes; Fetuses-at-risk; Gestational age bias; Survival analysis; WIC.