Researchers studying over 60,000 women in Copenhagen have identified a significant link between childhood weight and adult fertility. The study, published in JAMA Network Open, suggests that being overweight or obese during school years can drastically reduce the likelihood of having children by a woman's mid-30s.
A 42% reduction in fertility odds by age 34
The study found that weight patterns established as early as age six have long-term reproductive consequences. According to the report, women who followed an obese trajectory in childhood saw their chances of giving birth drop by 22% between the ages of 25 and 29.. This decline accelerates significantly as women enter their 30s, with the odds of childbirth falling by 42% for those aged 30 to 34.
Even women in the overweight category faced notable risks. The research indicates that women with an overweight trajectory were 26% less likely to give birth between the ages of 30 and 34 compared to those with a healthy childhood BMI. These findings suggest that poor reproductive health may begin much earlier in life than previously assumed.
Tracking 60,864 Copenhagen women via school health exams
To reach these conclusions, researchers examined data from 60,864 women born in Copenhagen between 1950 and 1989. The team utilized yearly school health examinations to track height and weight, then linked this information to the Danish Medical Birth Register to identify who eventually gave birth between the ages of 18 and 45.
This longitudinal approach allowed Dr. Dorthe Pedersen and her team at Copenhagen University Hospital to observe how early-life BMI patterns correlate with reproductive outcomes over several decades. While the study noted that 78% of the participants did give birth, approximately 22% of the group remained childless.
The 20% childlessness rate in Western Europe
These findings arrive amid a broader demographic shift across Western Europe. As reported by the study's authors ,approximately 20% of women born around 1965 in Western European countries remain childless, contributing to birth rates that have fallen below replacement levels.
This trend is increasingly viewed as "involuntary childlessness." While many assume people are choosing not to have children, estimates suggest only about 5% of European women aged 18 to 40 do not intend to become parents.. this reality is compounded by the fact that one in three school-aged girls in Europe is currently living with overweight or obesity, a statistic that may fuel future fertility declines.
The search for adult BMI and causal data
While the associations are striking, the researchers at Copenhagen University Hospital emphasize that this study does not establish a direct cause-and-effect relationship. A primary unknown is how adult BMI interacts with these early-life patterns; without more data on weight during the years women actually attempt to conceive, the full picture remains incomplete.
The exact biological mechanisms behind this link also remain unverified. Dr. Dorthe Pedersen suggested that the connection might be driven by physiological or social pathways, such as hormonal disturbances or impacts on egg quality,but more research is needed to confirm these specific drivers.
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