Childhood BMI and Childbirth: A Gap That Widens With Age

Pedersen and colleagues linked school health records to national registers for 60,864 Danish women born between 1950 and 1989, using measured height and weight from ages 6 to 15 to assign each girl to one of five latent-class childhood BMI trajectories - below average, average, above average, overweight, and obesity. Women were followed for a first childbirth from 1977 through 2022, and for hospital-based infertility diagnoses. Only 3.8% followed an obesity trajectory. Cause-specific hazard ratios compared each trajectory with the average trajectory, stratified by 10-year birth cohort and adjusted for parental education.

The association with childbirth was absent in early adulthood and deepened with age. For the obesity trajectory the adjusted hazard of giving birth was 0.99 (95% CI, 0.91-1.08) at ages 18-24, 0.78 (0.72-0.86) at 25-29, 0.58 (0.52-0.66) at 30-34, and 0.56 (0.46-0.67) at 35-45. The overweight trajectory followed a shallower version of the same curve (0.74 at 30-34). The below-average trajectory showed no association at any age. The pattern held, though attenuated, when analysis was restricted to ever-married women, arguing against partnership formation as the sole explanation.

The infertility results cut against a simple biological reading. Childhood BMI trajectories were not associated with any infertility (obesity adjusted HR 0.97; 95% CI, 0.85-1.10). The one signal - a lower hazard of primary infertility for the obesity trajectory (adjusted HR 0.79; 0.68-0.92) - is, as the authors caution, most plausibly an artifact of Danish policy that restricts publicly funded IVF above defined BMI thresholds, so heavier women may never be referred and never diagnosed. Fewer recorded births without a corresponding excess of diagnosed infertility is the tension the paper leaves open, and the exposure window closing at age 15 means adult BMI - the proximate variable during attempts to conceive - was never observed.