Maternal prepregnancy BMI and size at birth: race/ethnicity-stratified, within-family associations in over 500,000 siblings.

Abstract Purpose To estimate racial/ethnic-stratified effects of maternal pre-pregnancy BMI on size for gestational age at birth, by comparing siblings within families. Methods This study examined linked vital statistics and patient discharge data from 580,960 infants born to 278,770 women in the State of California (2007-2012). To control for family-level confounding, we used fixed effects multinomial regression, modelling size for gestational age (small [SGA], appropriate, large [LGA]) as a function of maternal BMI (underweight, normal weight, overweight, obesity class I, II, III) and time-varying covariates. We conducted overall and race/ethnicity-stratified (non-Hispanic white, black, Asian; Hispanic) analyses. For comparison, we fit analogous random effects models, which do not control for family-level confounding. Results In fixed effects models, maternal BMI was most strongly associated with LGA in non-Hispanic white women, reaching 6.7 times greater for class III obesity [OR (95% CI): 6.7 (5.1, 8.7)]; and weakest in black women [OR (95% CI): 3.0 (1.5, 5.7)]. Associations with SGA were similar across race/ethnicity. Compared to random effects estimates, fixed effects were most attenuated for LGA associations among racial/ethnic minority women. Conclusions Maternal pre-pregnancy BMI was differentially associated with size for gestational age across racial/ethnic groups, with the strongest family-level confounding in racial/ethnic minority women.