Although many public health analysts have found the Special Supplemental Nutrition Program for Women, Infants, and Children (WIC) to be strongly protects against pre-term birth, the clinical literature provides little support for such an association. One limitation is that most evaluations of WIC have focused almost exclusively on birth outcomes. And yet, if the association between WIC and enhanced infant health is truly causal, then participation should also be associated with indicators of maternal behavior that affect intrauterine growth and improve maternal health. Remarkably few attempts have been made to offer such corroborating evidence.
This study tested whether early and sustained participation in WIC during pregnancy results in less maternal smoking, greater weight gain, more breastfeeding, and healthier infants among women on WIC in North Carolina between 1996 and 2003.
The study used data from the Pregnancy Nutrition Surveillance System (PNSS) in North Carolina and New Jersey. The PNSS file in North Carolina had been further linked to birth certificates. North Carolina proved to be richer and to have fewer missing values than the PNSS data from New Jersey. Thus, the primary results focused on North Carolina. The data provided over 400,000 observations on WIC participants in North Carolina between 1996 and 2003 and 140,000 in New Jersey between 1999 and 2003.
The study questions whether increased exposure to WIC is associated with greater improvement in birth outcomes.
The analysis began by replicating the frequently reported finding that prenatal WIC participation strongly protects against pre-term birth. However, if WIC protects against adverse birth outcomes, then greater exposure to WIC should be associated with greater improvement in birth outcomes. As a test, the study compared the outcomes of women who enrolled in WIC in the first, second, and third trimester of pregnancy with women who enrolled postpartum. Postpartum enrollees were a potentially credible comparison group for prenatal participants. First, they are clearly eligible for WIC, and second, any stigma associated with publicly funded welfare programs was not sufficient enough to prevent participation. The study was also able to stratify the analyses by race and ethnicity. In addition, samples were sufficiently large to allow analysis of subsets of women in an effort to lessen heterogeneity among participants. Specifically, the study examined outcomes among women all of whom registered for prenatal care in the first trimester and who had no previous live births.
Despite these attempts to limit potential selection bias, one can never be sure that estimates are free of such contamination. Thus, the study used falsification exercises to uncover potential selection bias. For instance, the North Carolina PNSS asks women if they quit smoking before their first prenatal care visit. These women are referred to as spontaneous quitters. The study regressed spontaneous quitting on the timing of WIC enrollment among women who had a first birth and who initiated prenatal care in the first trimester. There should be no association between the cessation of smoking and the timing of WIC enrollment because quitting precedes enrollment. Should the study find, for example, that women who enroll in WIC early were more likely to “spontaneously quit,” such a finding would suggest that early enrollees are more health-conscious and more motivated than women who enroll later.
Findings represent a challenge to those who contend that WIC has a meaningful impact on birth outcomes.
The study was able to replicate the frequently reported finding that prenatal WIC participation protects against pre-term birth. However, when examining the association between WIC participation and pre-term birth by trimester of WIC enrollment, the study found that the association between prenatal WIC participation and lower rates of pre-term birth was driven primarily by women who enrolled in WIC in the third trimester. These were counterintuitive findings. The results pointed to fetal selection rather than strong treatment effects associated with WIC. As additional evidence, the study found no association between greater exposure to WIC and reduced smoking during pregnancy. In addition, women who enrolled in WIC in the first trimester were not more likely to breastfeed than women who enrolled postpartum. If WIC were to strongly protect against pre-term birth, then the study would have expected WIC to have a substantial effect on maternal behaviors, such as smoking and breastfeeding.
The results of the study should not come as a surprise. After decades of clinical trials aimed at preventing pre-term birth, few interventions have proven effective. It is unlikely, therefore, that a modest nutritional supplementation program could achieve what well-designed trials have not. The study concluded that a focus on birth outcomes is misguided. It recommends that the success of WIC be evaluated on how effectively programs achieve changes in maternal health and behavior that have well-established links to health over the lifetime of participants and their offspring. Obvious targets would be smoking cessation, breastfeeding, and obesity prevention.