Early Life BMI Trajectories and Associations With Childbirth and Infertility Among Women.

Importance: Concomitant with the global obesity epidemic, fertility has declined in Western countries with birth rates falling below replacement levels. A higher body mass index (BMI) among women is associated with reduced fertility, but associations with body size earlier in life are underexplored. Objective: To investigate whether trajectories of childhood BMI are associated with timing of first childbirth and infertility in women. Design, Setting, and Participants: This prospective, population-based cohort study used linked national register and school health examination data of women aged 18 to 45 years who were born in Copenhagen, Denmark, between 1950 and 1989. Participants were followed up for reproductive outcomes from January 1, 1977 to December 31, 2022. Analytic work was conducted from May to October 2025. Exposure: Measured height and weight data used to calculate childhood BMI trajectories from ages 6 to 15 years. Main Outcomes and Measures: Registration of childbirth and hospital-based diagnoses of infertility identified in the Medical Birth Register, the Danish Infertility Cohort, the National Patient Register, and the In Vitro Fertilization Register. Hazard ratios (HRs) and 95% CIs were estimated using Cox regression models. Results: This study included 60 864 women in the childbirth analysis, of whom 47 669 (78.3%) gave birth (median [IQR] age, 27.2 [23.7-30.9] years); 58 148 were included in the primary infertility analysis, of whom 5381 (9.3%) were diagnosed with primary infertility (median [IQR] age, 31.3 [27.9-35.1] years); and 63 066 were included in the any infertility analysis, of whom 6961 (11.0%) were diagnosed with any infertility (median [IQR] age, 31.9 [28.4-35.7] years). Women with an obesity trajectory in childhood had a lower hazard of giving birth than those with an average trajectory, and this hazard decreased with increasing age (ages 25-29 years: HR, 0.78 [95% CI, 0.72-0.86]; ages 35-45 years: HR, 0.56 [95% CI, 0.46-0.67]). Similarly, women with an overweight trajectory during childhood had a lower hazard of giving birth, which decreased with increasing age. No associations were observed between the childhood BMI trajectories and primary or any infertility. Conclusions and Relevance: In this cohort study of Danish women, associations with childbirth decreased from mid- to late-reproductive ages, when the window of fertility narrows. These findings suggest that childhood body size is associated with timing of childbirth, highlighting the importance of addressing women's reproductive health early in the life course.