Maternal education, birth weight, and infant mortality in the United States

Abstract

This research determines whether the observed decline in infant mortality with socioeconomic level, operationalized as maternal education (dichotomized as college or more, versus high school or less), is due to its "indirect" effect (operating through birth weight) and/or to its "direct" effect (independent of birth weight). The data used are the 2001 U.S. national African American, Mexican American, and European American birth cohorts by sex. The analysis explores the birth outcomes of infants undergoing normal and compromised fetal development separately by using covariate density defined mixture of logistic regressions (CDDmlr). Among normal births, mean birth weight increases significantly (by 27-108 g) with higher maternal education. Mortality declines significantly (by a factor of 0.40-0.96) through the direct effect of education. The indirect effect of education among normal births is small but significant in three cohorts. Furthermore, the indirect effect of maternal education tends to increase mortality despite improved birth weight. Among compromised births, education has small and inconsistent effects on birth weight and infant mortality. Overall, our results are consistent with the view that the decrease in infant death by socioeconomic level is not mediated by improved birth weight. Interventions targeting birth weight may not result in lower infant mortality.

Fig. 1

Graphical illustration of the Wilcox-Russell (Wilcox and Russell 1990) definition of causality. Panel a represents a shift in the birth weight distribution that is accompanied by an identical shift in the birth weight–specific mortality curve owing to a stressor, so that there is no change in the overall mortality and birth weight is not causal. Panel b represents a shift in birth weight that is not accompanied by an identical shift in the mortality curve, so that the stressor has an indirect effect on mortality and birth weight could potentially be on the causal pathway to infant mortality. Panel c represents a birth weight–independent change of the mortality curve (i.e., a vertical shift of the whole curve) owing to a stressor when birth weight is not causal, so that a direct effect of the stressor occurs

Fig. 2

Graphical illustration of covariate density defined mixture of logistic regressions (CDDmlr) without any exogenous covariate as applied to birth weight and infant mortality. Results presented are for European American males in 2001. The solid lines represent the total population, the dotted lines represent the primary subpopulation, and the dashed lines represent the secondary subpopulation. Panel a demonstrates the total distribution of birth weight as a weighted sum of two Gaussian densities (i.e., the primary and the secondary subpopulations). The secondary subpopulation is considered to consist of compromised births, because it accounts for most low birth weight and macrosomic infants. The primary subpopulation consists of the remaining births who are considered to be undergoing normal fetal development. Panel b demonstrates the total birth weight–specific mortality curve as a weighted sum of the primary and the secondary birth weight–specific mortality curves. Results for other five populations of interest are similar

Fig. 3

Birth weight densities by maternal education: European American males, 2001. In panels a–c, the solid lines represent low education, and the dashed lines represent high education. Corresponding fine lines are the bias-adjusted 95 % confidence intervals, which, when they overlap the predicted line, are not visible. Panel a represents the primary subpopulation, panel b represents the secondary subpopulation, and panel c represents the total population. Results for all six populations studied are similar. However, differences in birth weight distributions by maternal education for Mexican Americans are smaller compared with European American and African American births. Panel d is a Q-Q plot indicating the goodness of fit of the mixture model to the birth weight density. The example given is European American male births to mothers with high educational levels. The results for the other populations also indicate excellent fits

Fig. 4

Standardized birth weight–specific mortalities by maternal education: European American males, 2001. The solid lines represent low education, and the dashed lines indicate high education. Corresponding fine lines are the bias-adjusted 95 % confidence intervals. Panel a represents the primary subpopulation, panel b shows the secondary subpopulation, and panels c and d are for the total population. In panel d, the inverted triangles and the triangles are the observed birth weight–specific mortalities (estimated from binned data) for low and high education, respectively. Results for all six populations studied are similar. However, differences in mortality curves by maternal education are smaller for Mexican American births compared with European American and African American births