Stochastic Mediation Contrasts in Epidemiologic Research: Interpregnancy Interval and the Educational Disparity in Preterm Delivery

That conditioning on a common effect of exposure and outcome may cause selection, or collider-stratification, bias is not intuitive. We provide two hypothetical examples to convey concepts underlying bias due to conditioning on a collider. In the first example, fever is a common effect of influenza and consumption of a tainted egg-salad sandwich. In the second example, case-status is a common effect of a genotype and an environmental factor. In both examples, conditioning on the common effect imparts an association between two otherwise independent variables; we call this selection bias.

Adverse birth outcomes, such as preterm birth and low birth weight, have serious health consequences across the life course. Socioeconomic disparities in birth outcomes have not been the subject of a recent systematic review. The aim of this study was to systematically review the literature on the association of socioeconomic disadvantage with adverse birth outcomes, with specific attention to the strength and consistency of effects across socioeconomic measures, birth outcomes, and populations. Relevant articles published from 1999 to 2007 were obtained through electronic database searches and manual searches of reference lists. English-language studies from industrialized countries were included if (1) study objectives included examination of a socioeconomic disparity in a birth outcome and (2) results were presented on the association between a socioeconomic predictor and a birth outcome related to birth weight, gestational age, or intrauterine growth. Two reviewers extracted data and independently rated study quality; data were analyzed in 2008-2009. Ninety-three of 106 studies reported a significant association, overall or within a population subgroup, between a socioeconomic measure and a birth outcome. Socioeconomic disadvantage was consistently associated with increased risk across socioeconomic measures, birth outcomes, and countries; many studies observed racial/ethnic differences in the effect of socioeconomic measures. Socioeconomic differences in birth outcomes remain pervasive, with substantial variation by racial or ethnic subgroup, and are associated with disadvantage measured at multiple levels (individual/family, neighborhood) and time points (childhood, adulthood), and with adverse health behaviors that are themselves socially patterned. Future reviews should focus on identifying interventions to successfully reduce socioeconomic disparities in birth outcomes. 2010 American Journal of Preventive Medicine. Published by Elsevier Inc. All rights reserved.