Girls who followed overweight or obesity BMI patterns during childhood were less likely to give birth at some points in adulthood, according to a study that tracked nearly 61,000 women in Denmark.
The findings, which were published in JAMA Network Open, do not show that higher childhood BMI caused infertility. Women in the higher-BMI groups were not more likely to receive an infertility diagnosis, and the researchers could not determine whether differences in childbirth reflected biology, adult body weight, relationships, personal choices or other social factors.
Researchers linked childhood school health records with national birth and medical registries. The analysis included 60,864 women born between 1950 and 1989 who had their height and weight measured during school health examinations between ages 6 and 15.
The girls were grouped into five childhood BMI patterns: below average, average, above average, overweight and obesity. Researchers then identified births recorded between ages 18 and 45.
Overall, 78% of the women gave birth, at an average age of 27, while 22% had no recorded birth during the study period.
Women who followed overweight or obesity BMI patterns during childhood were less likely to give birth than those in the average group, particularly from their mid-20s onward.
Between ages 25 and 29, women in the childhood obesity group were 22% less likely to give birth than women in the average BMI group. From ages 30 to 34, they were 42% less likely to give birth.
Women in the childhood overweight group were 9% less likely to give birth between ages 25 and 29 and 26% less likely between ages 30 and 34.
Those comparisons describe differences in the rate of childbirth, not the percentage of women who were biologically unable to become pregnant.
The study did not find that childhood BMI patterns were associated with either primary infertility or any recorded infertility diagnosis. That distinction is important because not having a child by a certain age can happen for many reasons unrelated to reproductive capacity.
Some women may not want children. Others may not have a partner, may delay parenthood for personal or financial reasons or may face barriers to fertility care. Body-size stigma and social experiences could also influence relationships, health care and family formation.
The researchers did not have information about whether participants wanted children, were actively trying to become pregnant or considered themselves voluntarily or involuntarily childless.
“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 Dorthe C. Pedersen of Copenhagen University Hospital – Bispebjerg and Frederiksberg.
The results suggest an association, but they cannot establish that childhood body size itself reduced the likelihood of having a child.
One of the largest unanswered questions is the role of body weight in adulthood.
The main analysis did not include adult BMI, making it impossible to determine whether the association reflected a lasting effect of childhood growth or the tendency for higher childhood BMI to continue into adulthood.
Adult obesity can be associated with irregular ovulation, polycystic ovary syndrome, pregnancy complications and other conditions that may affect reproduction. Without adult measurements, the researchers could not separate those possible influences from childhood BMI.
The analysis also accounted for birth cohort and the highest educational level achieved by the participants’ parents, but many other factors could have influenced the results.
These include adult education, income, relationship history, menstrual health, mental health, pregnancy intentions, access to fertility treatment and other medical conditions.
Because this was an observational study, unmeasured differences among the groups may explain at least part of the association.
The historical setting also matters. Participants were born across nearly four decades, from 1950 through 1989. During that period, the average age of first birth, access to fertility treatment, social expectations and attitudes toward marriage and parenthood changed considerably.
The researchers accounted for birth cohort in their models, but the findings may not fully reflect the experiences of women reaching adulthood today.
The cohort was also drawn from Copenhagen and was largely white. The results may not apply to populations with different cultural backgrounds, health care systems or patterns of childhood growth.
Senior author Jennifer L. Baker said the findings point to the possibility that reproductive health is shaped by experiences and health patterns beginning long before someone plans a pregnancy.
“Our study paves the way for a better understanding of how overweight and obesity during childhood may contribute to involuntary childlessness,” Baker said. “More broadly, our findings suggest that reproductive health has much earlier origins than is commonly appreciated.”
The study, however, did not establish that the women without recorded births were involuntarily childless. It also did not identify the biological or social pathways that might connect childhood BMI with later childbirth.
Possible biological explanations could include hormonal or metabolic differences that continue into adulthood. Social explanations may include differences in partnering, stigma, socioeconomic circumstances or decisions about when and whether to have children.
Future studies will need adult body-weight data, clearer information about pregnancy intentions and measures of relationship and social circumstances to determine which explanations are most important.
The findings should not be used to suggest that a child’s future fertility can be predicted from BMI or that girls in larger bodies are destined to experience reproductive problems.
They instead show that childhood growth patterns were associated with later childbirth in this Danish population, while leaving the reasons for that difference unresolved.
The study was funded by the Independent Research Fund Denmark. Baker reported consulting for Novo Nordisk outside the submitted work, with all fees paid to her institution.