Women who followed overweight or obesity BMI trajectories during childhood were significantly less likely to give birth in adulthood than those with average BMI trajectories. Furthermore, this association became much more pronounced after the age of 30, highlighting a long-term reproductive impact.
However, researchers found that childhood BMI trajectories were not significantly associated with an increased risk of diagnosed infertility. This surprising nuance suggests the reasons for lower birth rates may involve biological, social, and behavioural factors beyond clinical infertility alone.
How Was This Large Study Conducted?
The study was led by Dorthe C. Pedersen, PhD, Center for Clinical Research and Prevention, Copenhagen University Hospital-Bispebjerg and Frederiksberg, Copenhagen, Denmark. It was published online in JAMA Network Open.
Researchers conducted a large prospective population-based cohort study to evaluate associations between childhood BMI trajectories and later childbirth and infertility outcomes in women. The study combined data from multiple Danish national registries to track reproductive health over several decades.
They analyzed data from women born between 1950 and 1989, including 60,864 women in the childbirth analysis with a median age at childbirth of 27.2 years. In addition, they included 58,148 women in the primary infertility analysis and 63,066 women in the any infertility analysis, with follow-up from January 1977 through December 2022.
Childhood BMI was calculated from height and weight measurements collected during routine school health examinations from 6 to 15 years of age. Consequently, researchers could classify women into five distinct BMI trajectory groups: below-average, average, above-average, overweight, and obesity.
What Did The Study Find About Childbirth Rates After 30?
After adjustments for other factors, women with obesity BMI trajectories in childhood were significantly less likely to give birth as they aged compared to those with average trajectories. This gap widened considerably as women moved into their thirties and forties.
For women with childhood obesity trajectories, the likelihood of giving birth was lower at 25-29 years with a hazard ratio of 0.78, at 30-34 years with a hazard ratio of 0.58, and at 35-45 years with a hazard ratio of 0.56.
Similarly, women with overweight trajectories in childhood were also less likely to give birth at 25-29 years with HR 0.91, at 30-34 years with HR 0.74, and at 35-45 years with HR 0.82 compared to average BMI peers.
Women with above-average BMI trajectories showed similar childbirth patterns at younger ages but were also less likely to give birth at 30-34 years and 35-45 years. Interestingly, no significant association with childbirth was observed among women with below-average BMI trajectories across any age group whatsoever.
Did Childhood BMI Increase Infertility Diagnoses?
This is where the findings become particularly interesting and counterintuitive for many readers. Women with obesity BMI trajectories during childhood were actually less likely to be diagnosed with primary infertility at ages 20-45 than those with average trajectories.
Specifically, they showed a hazard ratio of 0.79 for primary infertility, meaning a 21% lower likelihood of receiving a hospital diagnosis. Furthermore, no significant associations were observed between other childhood BMI trajectories and primary or any infertility outcomes.
Therefore, lower birth rates in this group do not appear to be fully explained by higher rates of medically diagnosed infertility alone.
Analysis: Why Does Childhood Weight Influence Future Childbirth?
This study fundamentally changes how experts view obesity and reproductive health across a woman’s entire lifespan. Traditionally, doctors have viewed obesity solely as a pregnancy-related comorbidity that increases risks during pregnancy itself.
However, these findings support a much broader concept that reproductive health may be shaped long before reproductive age actually begins. The effects of childhood BMI appear to be established early and become more visible when fertility difficulties naturally become more apparent after age 30.
Experts suggest several possible explanations beyond biology, including metabolic changes, hormonal imbalances, psychosocial factors, and differing life choices that influence childbearing decisions. Moreover, women with higher childhood BMI may face stigma or have fewer opportunities for early health counselling.
“By the time women reach their thirties, 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,” the authors wrote.
In an invited commentary, experts added, “Obesity should no longer be viewed solely as a pregnancy-related comorbidity, but rather a condition that influences the entire reproductive continuum, from conception to long-term offspring health.”
Q&A
Q: Does childhood obesity mean a woman will definitely be infertile?
A: No, the study found no increased risk of diagnosed infertility for most BMI groups, and women with childhood obesity trajectories were actually less likely to receive a primary infertility diagnosis.
Q: At what age does childhood BMI most affect childbirth chances?
A: The impact becomes most pronounced after age 30, with women who had obesity trajectories in childhood being 44% less likely to give birth between ages 35-45.
Frequently Asked Questions
1. Can childhood overweight affect your ability to have children later?
Yes, a large Danish study of over 60,000 women found that overweight and obesity trajectories from ages 6 to 15 are linked to lower rates of giving birth in adulthood, especially after age 30.
2. Does having obesity as a child increase infertility risk?
Surprisingly, the study found that women with childhood obesity trajectories were less likely to be diagnosed with primary infertility, suggesting other social or biological factors may explain lower birth rates.
3. What can parents do to protect future reproductive health?
Experts emphasize preventive efforts focused on healthy growth, balanced nutrition, regular physical activity, and healthy weight management well before reproductive age begins.
4. Is this study relevant for women today?
Yes, although participants were born between 1950-1989, the biological link between early-life BMI and reproductive health remains highly relevant, especially as childhood obesity rates continue to rise globally.
Final Word: This research proves that reproductive health is a lifelong journey that starts in childhood, not just at the time of trying to conceive.
Disclaimer: This article is for informational purposes only and does not constitute medical advice. Please consult your healthcare provider for personalized guidance.
