US Black vs White disparities in foetal growth: physiological or pathological?

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Abstract
Background Birthweight for gestational age is lower in US Black infants than in US White infants. It is unknown, however, whether this difference is ‘normal’ (i.e. physiological) or reflects pathological foetal growth restriction. Methods We applied an analytic approach based on foetuses at risk to compare gestational age-specific rates of live birth, ‘revealed’ small-for-gestational-age (SGA), and neonatal mortality among singleton infants ≥22 weeks of gestation and ≥500 g born in 1998–2000 to US White (n = 9 012 194), US-born Black (n = 1 554 382), and foreign-born Black (n = 200 395) mothers. Graphical methods and Cox proportional hazards regression analyses were used to compare outcomes in the three ethnic groups. Results Rates of live birth and neonatal mortality were highest at all gestational ages in US-born Blacks, lowest in Whites, and intermediate in foreign-born Blacks. The revealed SGA pattern cohered much more closely with the observed pattern for neonatal mortality when SGA was defined based on a single, overall standard of birthweight for gestational age than when based on ethnic group-specific standards. Conclusion The closer coherence of revealed SGA and neonatal mortality rates based on a single standard and the intermediate pattern among foreign-born Blacks strongly suggest that Black–White differences in birthweight for gestational age are pathological, rather than physiological.