August 31, 2026
Higher childhood BMI linked to lower birth rates in adulthood
Women on an obesity curve from age six were 42% less likely to give birth at 30 to 34, linking childhood weight to birth rates later in life
ByLuis Arrais
Author
Advancing age is the biggest known reason that women who want children end up without them. A new study of Danish women suggests a large part of the issue begins decades earlier. Girls who had obesity from age six through their teens were less likely to give birth in their late 20s, and less likely still after the age of 30
Below age 25, there was no difference at all. That puts the first sign years before the age at which most women learn they’re having trouble
Clinicians tracked girls’ BMI for years
Danish law required a health examination for every child in Copenhagen schools, public or private
Trained clinicians measured height and weight the same way each time, so none of this rests on what a grown woman remembers weighing at age 12. Those cards became the Copenhagen School Health Records Register
Dorthe C. Pedersen of the Center for Clinical Research and Prevention, and her colleagues, pulled 60,864 files, all from women born between 1950 and 1989. They followed those women through Denmark’s national birth and patient registers to the end of 2022
Most girls had three to ten measurements on record. The team sorted them into five weight paths, or trajectories: below average, average, above average, overweight and obese. Only 3.8% landed on the obesity path
In an interview with Earth.com, Pedersen was asked what parents and doctors should take from a child’s growth years
“First of all, parents should not be worried based on these results,” she said. “Our findings are population-based, so they tell us about patterns across groups and cannot be used to predict individual girls’ reproductive health.”
Higher BMI linked to fewer births
Between the ages of 18 and 24, women from every path had children at the same rate. Nothing separated them
From 25 to 29, that changed. Women who had followed the obesity path in childhood were 22% less likely to give birth than women from the average path. Women from the overweight path were 9% less likely
The differences increased with age. By ages 30 to 34, women with higher childhood BMI were 42% (with obesity) and 26% (overweight) less likely to give birth
The heavier the childhood path, the fewer the births. After 35, even women from the above-average path were 17% less likely to give birth
Birth rate gaps widened with age
The median first birth in this study came at 27. So the differences appear when most of these women were becoming mothers, and widen with age
Pedersen told Earth.com she hadn’t expected the differences to be largest in the oldest women. “This is a bit eye-opening,” she said, “since many women try to become pregnant around this age.”
Below-average girls were no less likely to have children. The team also checked weight at ages seven and 13, before and during puberty, with the same answer
Infertility diagnoses showed no link
The team ran the same comparison for infertility. Among 58,148 women who had not yet had a child, 5,381 were diagnosed at a hospital or fertility clinic, at a median age of 31. Childhood weight had no bearing on which women those were
Women from the obesity path were actually diagnosed slightly less often. Asked by Earth.com, Pedersen said she had expected the opposite
“If women with obesity are less likely to receive a diagnosis because they have less access to fertility assessment or treatment, this could make an association more difficult to detect in our data,” she said
BMI compares a person’s weight with their height. The Danish Fertility Society caps IVF at a BMI of 30 for women under 30, and 35 for women over that age. This is because women with extra weight face higher risks in pregnancy
A woman above the limit may never be referred, and an unreferred woman doesn’t appear in the register
Future studies could test that, she added, in populations where BMI doesn’t decide who gets fertility care
Marriage did not explain the birth gap
Two kinds of explanation fit these results, and this study can’t choose between them
The social one already has evidence behind it. Earlier work found that women who carried extra weight in childhood were less likely to marry or live with a partner, and thus less likely to give birth. So the team ran the numbers again using only the 41,485 women who married before 45
The differences got smaller. They didn’t disappear. For married women on the obesity path, the drop after 30 was still there
The other explanation is biological. In adult women, extra body fat is linked to changes in the hormone signals between brain and ovaries, and to poorer egg quality. Whether it does the same when overweight or obesity develops at age 8 rather than 28 is barely studied
Adult BMI remains a missing piece
The health cards stop at age 15. Childhood obesity often persists into adulthood, so many women on the obesity path were probably heavier as adults too
Nothing here separates the childhood pattern from the adult weight it became. Pedersen’s team names that limit first
Parental education was the only background factor the analysis could account for. Danish women with less education are less likely to seek help for infertility, so infertility in that group is probably undercounted
The study can link childhood weight to birth at a later stage. It can’t show that one caused the other
What links childhood BMI to births?
Pedersen said the next step is work on what actually produces the pattern. That work, she said, could show how best to support girls and women living with obesity
“We believe that a combination of biological, social and behavioral factors may be involved,” she said, “and it will be important to disentangle these pathways.”
Fertility rates worldwide have fallen from 4.8 children per woman in 1950 to 2.2 in 2021, by a 2024 analysis. In many countries, birth rates no longer replace the population
One in three school-aged girls in the United States and Europe now has overweight or obesity, and for many, higher weight will persist into adulthood
This study can’t see the four decades between a school health card and a birth record. Filling that in means weighing the same women through adulthood, into the years they spend trying to conceive
The full study was published in the journal JAMA Network Open
Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates
Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com
—–
