Objectives The current study examined how prepregnancy body mass index (BMI), gestational weight gain, and birth weight cluster between births within women and between women who are sisters. Methods Using data from the National Longitudinal Survey of Youth 1979 cohort, we utilized nested, multivariable hierarchical linear models to examine the correlation of these three outcomes between births (n = 6006) to women (n = 3605) and sisters (n = 3170) so that we can quantify the clustering by sibship and by woman for these three pregnancy-related outcomes. Results After controlling for confounding covariates, prepregnancy BMI (intraclass correlation (ICC) 0.24, 95% CI 0.16, 0.32), gestational weight gain (ICC 0.23, 95% CI 0.16, 0.31), and infant's birthweight (ICC 0.07, 95% CI 0.003, 0.13) were correlated between sisters. Additionally, all three outcomes were significantly correlated between births for each sister, suggesting that prepregnancy BMI (ICC 0.82, 95% CI 0.81, 0.83), gestational weight gain (ICC 0.45, 95% CI 0.42, 0.49), and birth weight (ICC 0.31, 95% CI 0.28, 0.35) track between pregnancies in the same woman. Conclusions for Practice The observed clustering both within women and between sisters suggests that shared genetic and environmental factors among sisters play a role in pregnancy outcomes above and beyond that of women's own genetic and environmental factors. Findings suggest that asking a woman about her sisters' pregnancy outcomes could provide insight into the possible outcomes for her current pregnancy. Future research should test if collecting such a family history and providing tailored clinical recommendations accordingly would be useful.
