The Special Supplemental Nutrition Program for Women, Infants, and Children (WIC) is a national program designed to provide supplemental foods, nutrition education, and referral to health services for pregnant and lactating women, infants, and children up to 5 years of age. WIC is administered by the US Department ofAgriculture (Food and Nutrition Services) and is managed at the state level by health departments. It provides services to 7.4 million women and children annually in the United States; of these participants, approximately 11% are pregnant women.' The major goal ofWIC is to improve maternal and infant health. Eligibility for the program is based on income and nutritional risk.Studies have shown that participation in WIC by pregnant women is associated with a lower incidence oflow-birthweight, very-lowbirthweight, and preterm deliveries, especially among women at high risk owing to sociodemographic characteristics or medical conditions,29 and that participation is a cost-effective intervention for preventing low-birthweight deliveries.440I3 However, most ofthese studies did not distinguish between the 2 components of low biihweight, prmatuity (less than 37 weeks ofgestation at birth) and small-for-gestationalage deliveries. Many ofthe risk behaviors associated with preteni and small-for-gestational-age deliveries (and targeted by WIC interventions), such as maternal weight gain and maternal smoking, are more strongly associated with the latter type of delivery.'4 5Previous studies on WIC participation and birth outcome have usually included women who delivered prematurely. If a premature delivery occurs before enrollment, a woman has no chance of being included in WIC during her pregnancy. This selection bias may result in an overestimation ofthe effect of WIC on birth outcomes and could explain the strong effect ofWIC observed in these studies. To avoid this selection bias, we restricted our analysis to women who delivered full-term infants. We examined whether there is a dose-response association between length of WIC enrollment and small-for-gestationalage births, along with birthweight distributions according to length of enrollment in WIC. Methods Study PopulationIn 1992, the Michigan WIC program served 175 341 people per month, 13% of whom were pregnant women.'6"17 An eligible individual in Michigan must live in a household with an income at or below 185% of the federal poverty level and have one or more nutritional risk factors (e.g., inadequate diet or anemia). Our target population included pregnant women who were participating in WIC programs as well as those nonparticipants identified from birth certificate data as being income eligible on the basis of having delivery costs covered by Medicaid.

Collocation of WIC clinics at MCO sites can improve health care of low-income infants. However specific procedures for cooperation between WIC staff and other MCO staff are required to achieve this benefit.