Parental characteristics can explain why Japanese women give birth to the smallest infants in the United States

Abstract

Background: Whether birthweight differences observed between races are due to modifiable factors remains controversial. In the United States, Asian infants weigh less than white infants and Japanese infants weigh the least.Methods: Using US Natality data, we evaluated 4 132 319 singleton term livebirths to parents of non-Hispanic white or Japanese race/ethnicity from 2009 to 2012. Infants were categorised by parental race/ethnicity (both white, n = 4 116 637; Japanese father/white mother, n = 2377; white father/Japanese mother, n = 7478; both Japanese, n = 5827). We used multivariable regression to sequentially adjust for maternal characteristics to determine to what extent they explained differences in fetal growth due to maternal race/ethnicity.Results: Infants born to Japanese mothers were smaller by 132 (95% CI 101, 122) grams, with higher risk of small for gestational age (relative risk ratio (RRR) 1.56 (95% CI 1.47, 1.65) and lower risk of large for gestational age RRR 0.49, 95% CI 0.44, 0.54), compared to infants of white mothers. Differences in social factors and in maternal age, parity, and gestational age only minimally explained this difference. However, additionally adjusting for maternal height, body mass index and gestational weight gain substantially attenuated this difference to 20 (95% CI 12, 29) grams in birthweight, 0.91 (95% CI 0.86, 0.97) times risk of small for gestational age, and 1.06 (95% CI 0.96, 1.17) times risk of large for gestational age, with the effect similar across strata of paternal race/ethnicity.Conclusion: Differences in fetal growth between infants from Japanese and white mothers could be explained by differences in maternal height, pre-pregnancy weight, and gestational weight gain. Fetal growth potential appears to be similar across racial/ethnic groups when parental sizes are similar.