Body mass index in early adulthood and transition to first birth: Racial/ethnic and sex differences in the United States NLSY79 Cohort.

Studies show that body mass index during early adulthood ('early BMI') predicts the transition to first birth, but early childbearers tend to be omitted from such studies. This sample selection distorts the prevalence of childlessness, and particularly the racial/ethnic heterogeneity therein, because first birth timing differs by race/ethnicity. We imputed pre-parenthood early BMI for a larger sample, including early childbearers, for the same United States NLSY79 data used in a previous study and simulated differences in the probability of childlessness at age 40+ using posterior distributions based on the Bayesian framework. Obesity was consistently associated with higher childlessness across racial/ethnic groups in both sexes, but only among obese women were first births delayed until after early adulthood. The overall lower childlessness among the underweight women appeared largely driven by Black women. Our findings on the intersectionality of race/ethnicity and sex in the BMI-childlessness pathways encourage research on the underlying mechanisms and on more recent cohorts across different societies.