A study of almost 61,000 women in Denmark, published in the journal JAMA Network Open, finds that women who had an overweight or obesity body mass index (BMI) trajectory during childhood were less likely to have children than women with a normal (average) BMI trajectory, and this effect on childlessness was already apparent in their mid-20s.
"Our findings suggest that less favourable BMI patterns from as young as 6 years old may increase a women's risk of childlessness from their mid-20s onwards," said lead author Dr Dorthe C Pedersen from the Copenhagen University Hospital – Bispebjerg and Frederiksberg, Copenhagen, Denmark.
"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% of women born around 1965 remain childless [1], with birth rates falling below replacement levels. However, estimates suggest that only about 5% of European women aged 18–40 years do not intend to have children, indicating that most childlessness is likely involuntary [2].
It is well known that a higher 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.
To explore this further, researchers analysed data for 60,864 women from the Copenhagen School Health Records Register (including nearly every girl attending school in Copenhagen) born between 1950 and 1989 (aged 35 to 76 years now), who had information on height and weight measured at yearly school health examinations from aged 6 to 15 years. All were alive at age 18, did not emigrate, and were not lost to follow-up.
The women were grouped based on five distinct childhood BMI trajectories (modelled to capture childhood growth changes) between ages 6 and 15 years: below-average (13,460 girls; 22%), average (22,608; 37%), above-average (15,431; 25%), overweight (7,060; 12%), and obesity (2,305; 4%).
Researchers identified girls who later gave birth between the ages of 18 and 45 years by linking with the Danish Medical Birth Register (1977–2022).
Across the study period, 78% (47,669) women gave birth (at an average age of 27 years), and 22% remained childless (13,195).
After adjusting for potentially influential factors, including birth cohort and highest achieved parental educational level, women with an overweight or obesity trajectory from as young as 6 years old were less likely to give birth compared to women with an average childhood BMI trajectory, particularly at older ages.
For example, between the ages of 25 and 29 years, women with an obesity trajectory in childhood were 22% less likely to give birth than women with an average childhood BMI trajectory, and were 42% less likely to give birth at ages 30–34 years.
Similarly, women with an overweight trajectory were 9% less likely to give birth between the ages of 25 and 29 years than women with an average childhood BMI trajectory, and 26% less likely to give birth between the ages of 30 and 34 years.
According to Dr Pedersen, "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 from Copenhagen University Hospital – Bispebjerg and Frederiksberg, Copenhagen, Denmark, and President Elect of the European Association for the Study of Obesity (EASO) 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. Recognizing 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, can't 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, and further research to enhance the generalisability of the results beyond white populations and to more recent cohorts with evolving treatments, reproductive and partnering practices.
