The Use of Twins To Understand the Effect of WIC on Birth Outcomes

Year: 2004

Research Center: Institute for Research on Poverty, University of Wisconsin-Madison

Investigator: Joyce, Ted, and Diane Gibson

Institution: Baruch College, City University of New York

Project Contact:
Ted Joyce
Department of Economics and Finance
Baruch College, City University of New York
Box 10-225
One Bernard Baruch Way
NY, NY 10010
Phone: 212-802-5962


There is little evidence in the clinical literature to suggest that supplemental nutrition for pregnant women in developed countries has an important effect on birth outcomes. By contrast, studies in the social science literature indicate that the Supplemental Nutrition Program for Women, Infants, and Children (WIC) has had a major impact on the incidence of preterm birth. Clinicians view this finding as suspect, since few interventions tested in randomized trials have proven effective at preventing preterm birth. This study takes a first step towards reconciling the belief among policy analysts of the efficacy of WIC with the skepticism among medical researchers that nutritional supplementation in the United States is a meaningful determinant of birth outcomes.

The original study objective was to analyze the effect of prenatal WIC participation on birth outcomes among twins. However, during the research, it became evident that the overall association between prenatal WIC participation and birth outcomes had weakened considerably in New York City between 1988 and 2001. Accordingly, the scope of the project was expanded to analyze why the association between WIC and infant health among all births had become less robust over time. The analysis of twins was an important auxiliary project to support this larger research question.

Associations in the social science studies between WIC and preterm birth may be due to omitted variable bias. Therefore, this research considered measures of fetal growth in addition to preterm birth as an outcome.

Second, cross-sectional birth certificate data were utilized to examine the extent to which the association between WIC and birth outcomes had changed over time, and if so, whether the changing composition of participants could explain the difference. The growth in WIC coincided with the expansion in Medicaid eligibility thresholds that occurred in the late 1980s and early 1990s. Women on Medical Assistance are automatically eligible for WIC, even if the income thresholds for Medicaid exceed those for WIC. As a result, the growth in WIC among pregnant women was likely to include proportionately fewer women at risk for adverse birth outcomes. With 14 years of data and over 800,000 births to women on Medicaid in New York City, the data provided a unique opportunity to test whether the changing composition of women on Medicaid and WIC from the period before the Medicaid eligibility expansions through the most recent expansions for pregnant women under State Children's Health Insurance Program (SCHIP) could explain the weakening association with infant health.

Third, many studies reported that improvements in birth outcomes associated with WIC were greater among women at medical risk such as smokers, teenagers and those with a previous premature delivery. Such results were interpreted as evidence that WIC was more beneficial for women in need of nutritional assistance. But these risk factors tended to be correlated with other, harder to measure, determinants of birth outcomes such as substance abuse, sexually transmitted disease, and stressful home environments.

Effects associated with WIC among a sample of women who smoke, for instance, may reflect greater unobserved heterogeneity between WIC and non-WIC participants than was found among lower-risk groups such as nonsmokers. As a potentially less biased test of whether the association between WIC and birth outcomes was greater among women at nutritional risk, the analysis compared outcomes between WIC and non-WIC participants who deliver twins. Multiple gestations represent a random health shock that increases the risk of anemia, inadequate weight gain and adverse birth outcomes, but should be orthogonal to other risky behaviors.

Finally, a universal concern among WIC analysts has been selection bias. Do women who participate in WIC differ from non-participants in ways that are hard to measure but that are correlated with the outcomes of interest? As with almost all previous studies, this research lacked a truly exogenous instrument or quasi-experimental design to address selection bias directly. However, the analysis allowed the effects of WIC on fetal growth to vary over time and within relatively homogenous groups of women. Therefore, treatment effects that were clinically implausible and differed substantially over time and across groups should be interpreted cautiously and viewed as possible evidence of selection bias.

Findings indicate no statistical association between WIC and fetal growth, except for a positive association among US-born blacks who deliver twins. One implication may be that targeted nutritional supplementation during pregnancy might be useful. The broadly held notion that WIC improves birth outcomes emanates from its association with preterm birth, which the clinical literature has suggested is implausible. The analysis showed that prenatal WIC participation had at best a modest impact on fetal growth. Previous assessments of WIC by social scientists have tended to overlook the rather weak association with fetal growth.