Doctoral Candidate George Mason University Fairfax, Virginia, United States
Objectives: To estimate the impact of the 2009 revisions to the Supplemental Nutrition Assistance Program for Women, Infants and Children (WIC) food packages on breastfeeding outcomes using 11 years of National Immunization Survey – Child data and a combination of methods designed to address challenges encountered in estimating causal effects with observational data.
Methods: Our five breastfeeding outcomes were any breastfeeding, breastfeeding through 6 months, exclusive breastfeeding through 6 months, duration of breastfeeding (days), and duration of exclusive breastfeeding (days). To estimate treatment effects, we used an intent-to-treat regression-adjusted difference-in-difference (DID) model with propensity score weighting and state-level fixed effects. We defined our treatment group as children eligible for WIC (n=90,141) and our control group as children marginally ineligible for WIC (n=5,603) based on household income. Our approach sought to disentangle any causal effect of the 2009 revisions from overall trends in breastfeeding while minimizing biases that commonly affect observational studies of WIC.
Results: We observed improvements in four of the breastfeeding outcomes among children in the treatment group (ever breastfed, 4.6 percentage points, p< 0.001; breastfed through 6 months, 2.5 percentage points, p=0.003; exclusively breastfed through 6 months, 4.5 percentage points, p< 0.001; and duration of exclusive breastfeeding (days), 10 days, p< 0.001) and in one of the breastfeeding outcomes among children in the control group (ever breastfed, 7.1 percentage points, p< 0.001). However, none of the DID estimates were significant for any of the breastfeeding outcomes.
Conclusions: We found no evidence that the 2009 revisions had a causal effect on any of five breastfeeding outcomes among a large sample of children income-eligible for WIC. Any positive results observed in this study are likely a reflection of overall upward trends in breastfeeding behaviors that occurred in the United States before and after the 2009 revisions.
