Abstract
BackgroundInfant mortality has traditionally been analyzed as a function of birth weight and birth weight-specific mortality. Often, however, when comparing two populations, the population with higher overall mortality has lower mortality at low birth weights and a reversed pattern at higher birth weights. Methods standardizing birth weight, such as the "relative birth weight", have been proposed to eliminate these crossover effects, but such methods do not account for the separate contributions to birth weight of gestational age and fetal "growth."MethodsUsing data for singleton U.S. Blacks (n = 3,683,572) and Whites (n = 18,409,287), we compared neonatal mortality, gestational age, and the difference between the observed birth weight and the optimal birth weight (the weight at which neonatal mortality was lowest) among Black and White infants at the same relative birth weight.ResultsAt relative birth weights below zero, gestational ages were, on average, 2.4 ± 1.5 (mean ± standard deviation) weeks shorter for Blacks than for Whites for the same relative birth weight. At relative birth weights above zero, no differences were observed in gestational age, but the optimal birth weight occurred at a much higher relative birth weight in Whites than in Blacks (4150 vs. 3550 g).ConclusionsOur results suggest that comparisons of neonatal mortality between groups using "relative" birth weight may be potentially biased by differences in gestational age at low birth weights, and by the distance from the optimal birth weight at higher birth weights.
Highlights
Infant mortality has traditionally been analyzed as a function of birth weight and birth weight-specific mortality
We examine the association between neonatal mortality, race, and birth weight, considering relative birth weight, gestational age, and optimal birth weight
We show that the association between neonatal mortality and race-specific relative birth weight is severely confounded by gestational age and associated with distance from the racespecific optimal birth weight
Summary
Infant mortality has traditionally been analyzed as a function of birth weight and birth weight-specific mortality. Canada [4] and most other developed countries [5] have witnessed a dramatic reduction in infant mortality over the last quarter century This reduction has been almost entirely due to decreases in birth weight-specific mortality, largely attributable to improvements in access to, and quality of, obstetric and neonatal care. Despite the utility of this compartmentalization, comparisons of birth weight-specific mortality (e.g., between racial groups or between smokers and nonsmokers) often reveal an interesting "paradox": mortality rates among LBW infants born to Black mothers (or to smokers) are lower than those born to White mothers (or nonsmokers). Some investigators have developed statistically sophisticated models based on "relative birth weight," i.e., birth weight relative to the mean birth weight of the relevant population (e.g., Blacks or smokers) [9,10,11,12,13,14]
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