Women with higher childhood BMI trajectories were less likely to give birth from their mid-20s onward, yet researchers found little evidence that diagnosed infertility explained the pattern.
Study: Early Life BMI Trajectories and Associations With Childbirth and Infertility Among Women. Image Credit: Piyawat Nandeenopparit / Shutterstock
A recent study published in the journal JAMA Network Open reports associations between childhood overweight or obesity in women and lower hazards of childbirth in the later reproductive years, compared to women with average body mass index (BMI) trajectories in childhood. Conversely, childhood BMI trajectories were generally not associated with diagnosed infertility.
These findings suggest that childhood BMI may be associated with the timing of first childbirth, particularly in later reproductive years.
Background
In 2021, estimates showed that on average 2.2 children were born per woman, compared to 4.8 in 1950. In many countries, fertility rates are below the population replacement threshold (2.1 children per woman).
In high-income countries, the proportion of childless women is on the rise. In the United States (US), for instance, it increased from 10% among women born in 1945 to 15% among those born in 1970. Conversely, in Europe, only about 5% of women aged 18 to 40 are estimated not to intend to have children, suggesting that much childlessness may be involuntary. Moreover, nearly 7.5 million European women of reproductive age are infertile, unable to conceive despite 12 months of regular unprotected intercourse.
Advancing age is a major contributor to childlessness and infertility, while overweight and obesity can further increase these risks and now affect about one in three school-aged girls in the US and Europe. The long-term reproductive impact of high childhood BMI remains unclear, with existing studies yielding mixed results.
The current study sought to identify associations between BMI trajectories in childhood and childbirth and infertility in adult women.
Study characteristics
All study participants were women born in Copenhagen between 1950 and 1989, aged 18-45 years during the follow-up period. Reproductive outcomes were assessed from 1977 through 2022.
The study included 60,864 women in the childbirth analysis cohort, versus 58,148 women assessed for primary infertility associations with childhood BMI, and 63,066 assessed for any infertility. Overall, most women had an average BMI trajectory in childhood, while about 3.8% had obesity.
Women who had overweight or obesity BMI trajectories in childhood tended to have been born in later birth cohorts than those with an average BMI trajectory.
Childbirth analysis
In this cohort, 47,669 women (78%) gave birth at a median age of 27 years. The hazard of childbirth in relation to high childhood BMI varied across age groups.
In the 18-24-year age group, women with obesity or overweight in childhood had similar childbirth hazards as those with average BMI (the reference group).
At 25-29 years, such women had a lower hazard of childbirth than the reference group, with lower hazards also seen at older ages. For instance, at 25-29 years, the hazard of childbirth was 9% and 22% lower among women with childhood overweight and obesity, respectively, compared with the reference group.
After 30 years, the hazard of childbirth was markedly lower in women with either childhood overweight or obesity compared to the reference group. In the 35-45-year age group, it was 44% lower among the childhood obesity group compared to the reference group.
Women with an above-average BMI trajectory in childhood also showed lower childbirth hazards at older ages. After similar hazards at ages 18-24 and 25-29 years, the hazards were lower by 9% and 17% in the 30-34-year and 35-45-year age groups, respectively.
There was no difference between those with average and below-average childhood BMI across any age bracket.
Primary infertility analysis
Of the total cohort for this outcome, 5,381 women (9%) were diagnosed with primary infertility at a median age of 31 years. An obesity trajectory in childhood was linked to a 21% lower hazard of primary infertility between 20 and 45 years, which the authors suggested may reflect restricted access to fertility care rather than a biological reduction in infertility risk.
Notably, Danish guidelines restrict access to in vitro fertilization (IVF) for women with a BMI above 30 if younger than 30 years, or a BMI of 35 if older than 30 years. The authors therefore considered this finding one that should be interpreted with great caution.
None of the other BMI trajectories showed any association.
Any infertility analysis
In this cohort, 6,961 women were diagnosed with infertility at a median age of 32 years. No association was found between childhood BMI trajectory and recorded infertility during ages 20 to 45 years.
Importance of the study
The findings suggest that childhood BMI in women may be associated with a lower hazard of first childbirth, particularly in the later reproductive years, with both physiological and social mechanisms potentially contributing.
Limitations
This large prospective, population-based cohort study, based on national registry data and school health examination records from Denmark, supports the robustness of the findings.
This study did not directly examine the mechanisms underlying such associations, which remain underinvestigated. Women with lower educational attainment are less likely to seek medical help for infertility, possibly resulting in an underestimate of diagnosed infertility in this group.
Other potential confounders might have affected the results. Adult BMI data were unavailable, hindering the assessment of the effect of excess body fat on conception during the prime reproductive years.
Conclusion
The study found lower hazards of childbirth associated with childhood overweight or obesity from the mid- to late-reproductive years, with the associations particularly apparent after age 30. These patterns became more apparent with advancing age.
In contrast, childhood BMI trajectories were generally not associated with diagnosed primary infertility or any infertility, even though women with childhood obesity were less likely to have a recorded diagnosis of primary infertility.
These associations emerged as early as the relatively young 25-29-year age group, well before most women typically seek evaluation or treatment for infertility, and were more pronounced in the later reproductive years.
Overall, the authors suggest that “the influence of elevated childhood BMI on reproductive outcomes may begin early in life. By the time women reach their 30s, when fertility difficulties often become apparent, these effects may already be established, underscoring the need for preventive efforts focused on healthy growth and weight management well before the reproductive age."
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