Being overweight or obese as a child harms girls' fertility and slashes chances of becoming a mum, study suggests

By SHAUN WOOLLER, EXECUTIVE HEALTH EDITOR Published: 16:36, 4 August 2026 | Updated: 16:37, 4 August 2026 Being overweight or obese at school harms girls’ fertility and dramatically cuts their chances of becoming a mum, a study suggests. Those who piled on the most weight from the age of six were least likely to go on to have a family of their own by their mid-30s, with their odds slashed by up to 42 per cent. Researchers say their findings show poor reproductive health starts at an earlier age than commonly thought and more must be done to tackle a growing cause of ‘involuntary childlessness’. Lead author Dr Dorthe Pedersen, from Copenhagen University Hospital, Denmark, said: ‘Our findings suggest that less favourable BMI patterns from as young as six years old may increase a woman’s risk of childlessness from their mid-20s onwards. ‘By the time women reach their 30s, when many women start trying to conceive and fertility difficulties often become evident, the effects on reproductive capacity may already be established, underscoring the importance of preventive efforts focused on healthy growth and development in childhood.' In western European countries, about 20 per cent of women born around 1965 remain childless, with birth rates falling below replacement levels. However, estimates suggest that only about 5 per cent of European women aged 18 to 40 years do not intend to have children, indicating that most childlessness is likely involuntary. It is well known that a higher body mass index (BMI) among women is associated with a lower reproductive capacity, but there has been limited research examining the contribution of BMI in early life to childlessness and infertility in adulthood. Girls who piled on the most weight from the age of six were least likely to go on to have a family of their own by their mid-30s. To explore this further, researchers analysed data for 60,864 women born in Copenhagen between 1950 and 1989 who had information on height and weight measured at yearly school health examinations. They identified girls who later gave birth between the ages of 18 and 45 years by linking with the Danish Medical Birth Register. Across the study period, 78 per cent of women (47,669) gave birth - at an average age of 27 years - and 22 per cent remained childless (13,195). After adjusting for potentially influential factors, such as whether participants were in a relationship, women with an overweight or obese trajectory from as young as six years old were less likely to give birth compared to women with a healthy childhood BMI trajectory, particularly at older ages. For example, between the ages of 25 and 29 years, women with an obese trajectory in childhood were 22 per cent less likely to give birth than women with an average childhood BMI trajectory, and were 42 per cent less likely to give birth at ages 30 and 34 years. Similarly, women with an overweight trajectory were 9 per cent less likely to give birth between the ages of 25 and 29 years than women with an average childhood BMI trajectory, and 26 per cent less likely to give birth between the ages of 30 and 34 years. Dr Pedersen, who published her findings in the medical journal JAMA Network Open, said: ‘Although advancing age is a major driver of childlessness, with postponed parenthood being a substantial contributor, overweight and obesity appear to further increase these risks through physiological and social pathways, possibly including effects on egg quality and hormonal disturbances. ‘So it is concerning that one in three school-aged girls in Europe are currently living with overweight or obesity.’ Senior author Dr Jennifer Baker, also from Copenhagen University Hospital and president elect of the European Association for the Study of Obesity, added: ‘Our study paves the way for a better understanding of how overweight and obesity during childhood may contribute to involuntary childlessness. ‘More broadly, our findings suggest that reproductive health has much earlier origins than is commonly appreciated. ‘We need to be thinking of infertility as a cumulative physical and social experience, with effects beginning many years before women wish to become pregnant. ‘Recognising these early-life influences could reshape how we understand reproductive health and identify opportunities to support it long before family planning begins.’ The authors acknowledge that the findings are associations only, and as such, cannot establish a causal relationship between childhood BMI trajectories and childlessness. They point out several caveats including the need for additional data on BMI in adulthood to understand the importance of overweight and obesity when trying to conceive.