What explains away the increased risk of histological chorioamnionitis in African-American mothers of very-low-birthweight infants?

We sought explanations for African-American mothers' increased risk of chor ioamnionitis by sequentially adjusting for confounder variables both indivi dually and in groups. We searched for a subset of covariates that had the m ost influence on the chorioamnionitis odds ratio (OR) of these women. The s ample consisted of 305 African-American and 520 White mothers who gave birt h to a very-low-birthweight (less than or equal to 1500 g) infant between 1 991 and 1993, whose placenta was examined according to protocol and whose h ospital chart was reviewed. Histologically proven chorioamnionitis was pres ent in 43% of the placentas from African-American women and in 27% of those from Whites (crude OR 2.1, 95% confidence interval 1.5, 2.8). Singleton st atus appeared to be the most important effect modifier, with significant cr ude ORs of 1.5 among singletons and 3.4 among non-singletons. Using logisti c regression models in the whole sample and in subgroups, we sought to 'exp lain away' this increased risk. Indeed, addition of information about confo under variables resulted in considerable reduction in the ORs to 1.1 among singletons and 1.9 among non-singletons. Particularly important among the c onfounders were singleton birth, Medicaid insurance, duration of ruptured m embranes and gestational age. We discuss the possibility that this set of c onfounding variables conveys, in part, the same information as the variable African-American, and also perhaps information about the availability and/ or utilisation of prenatal health care.