Women with overweight or obesity body mass index (BMI) trajectories during childhood had lower hazards of giving birth beginning at ages 25 to 29 years, with stronger associations at older ages, according to a prospective, population-based cohort study published in JAMA Network Open.
Investigators analyzed linked school health examination and national registry data from women born in Copenhagen, Denmark, between 1950 and 1989. Participants were followed for reproductive outcomes through December 2022. Height and weight measured by trained school clinicians were used to calculate BMI trajectories from ages 6 to 15 years. Participants with at least 2 BMI measurements were included, and 90% had 3 to 10 measurements. Investigators categorized trajectories as below average, average, above average, overweight, or obesity.
Childbirth, which included live births and stillbirths, was identified through the Medical Birth Register. Investigators evaluated infertility diagnoses separately among women who had not previously given birth and among women regardless of previous childbirth. Cox proportional hazard models were stratified by 10-year birth cohort and adjusted for parental educational attainment. Because the proportional hazards assumption was not met for childbirth, investigators reported estimates across 4 reproductive age groups.
Among 60,864 women in the childbirth analysis, 47,669 (78%) gave birth. At ages 18 to 24 years, childbirth did not differ between the overweight or obesity trajectory groups and the average trajectory group in adjusted analyses. Beginning at ages 25 to 29 years, overweight and obesity trajectories were associated with lower hazards of childbirth, with the associations becoming more pronounced at older ages. Women with an above-average trajectory had lower hazards of childbirth at ages 30 to 34 and 35 to 45 years.
“Notably, we observed associations emerging as early as ages 25 to 29 years, well before most women typically seek evaluation or treatment for infertility,” wrote lead author Dorthe C. Pedersen, PhD, of the Center for Clinical Research and Prevention, Copenhagen University Hospital–Bispebjerg and Frederiksberg in Denmark, and colleagues.
Among 58,148 women evaluated for primary infertility, 5,381 (9%) received a diagnosis. Overall, childhood BMI trajectories were not associated with primary infertility, although women with an obesity trajectory had a lower hazard of primary infertility. Among 63,066 women evaluated for any infertility, 6,961 (11%) received a diagnosis, and childhood BMI trajectories were not associated with this outcome.
Analyses using BMI status at ages 7 and 13 years produced findings consistent with the main analyses for childbirth, primary infertility, and any infertility. Associations between childhood BMI trajectories and childbirth were also similar among 41,485 women who had ever married; associations for the obesity trajectory were attenuated but remained statistically significant.
The investigators noted that women with lower educational attainment were less likely to seek medical help for infertility and that infertility among women with lower socioeconomic position may therefore have been underestimated. Other limitations included potential residual and unmeasured confounding and the absence of adult BMI data. Investigators cautioned that the lower hazard of primary infertility among women with an obesity trajectory may reflect differences in access to fertility care. They noted that Danish guidelines restrict access to in vitro fertilization above specified adult BMI thresholds and that childhood obesity often persists into adulthood.
Disclosures: Coauthor Jennifer L. Baker, PhD, received grant funding from the Independent Research Fund Denmark during the study and consulting fees from Novo Nordisk that were paid to her institution outside the submitted work. No other disclosures were reported. The Independent Research Fund Denmark also funded the study through a grant awarded to co-author Jennifer L. Baker, PhD.
Source: JAMA Network Open
