Maternal obesity in pregnancy tied to higher offspring death rates into adulthood in 2.6 million births

Doctors have known for years that a baby born to a mother with obesity faces somewhat higher risks around birth: more complications in pregnancy, more stillbirths, more deaths in the first year. What happens after infancy has been close to unknown, because answering the question means following children for decades, and few countries keep the records to do it.

Sweden does. Hui Wang, Krisztina László and colleagues at Karolinska Institutet used the country’s birth register to follow more than 2.6 million people from the day they were born, some of them into their early forties. Their study, published in PLOS Medicine, found that the higher a mother’s body mass index, or BMI, at the start of pregnancy, the higher the death rate among her children, and that the pattern persisted across infancy, adolescence and adulthood but was absent during childhood. The authors are careful about what that means, and their own analysis of cousins shows why.

How Swedish birth records were used to study maternal BMI

Since 1982 Swedish midwives have recorded each woman’s weight at her first prenatal appointment, which for nine in ten women falls within the first 14 weeks. In the study’s words, weight was measured in light clothing at the first prenatal visit, so it reflects a woman’s size before pregnancy has changed it much, and it does not depend on anyone’s memory.

The researchers took every live birth from 1982 to 2014 with a usable BMI, 2,606,684 in all, and linked each child through national registers to the record of deaths up to the end of 2023. Mothers were grouped by the World Health Organization’s categories: underweight, normal weight, overweight, and three grades of obesity.

They adjusted the comparison for the mother’s age, education, income, country of birth and marital status, for smoking in early pregnancy, for psychiatric and cardiovascular illness before the birth, and for the child’s sex, birth order and year of birth. Over the study period the share of pregnant women who were overweight increased from 10.7% in 1982 to 25.2% in 2014, and the share with obesity rose from 2% to almost 13%.

How much higher was offspring mortality with maternal overweight and obesity?

Death in the first four decades of life is uncommon in Sweden. Over a median follow-up of 22 years, 21,981 of the 2.6 million died, fewer than one in a hundred. A little over a third of those deaths came in infancy and about four in ten in adulthood.

Within that small total, the rate climbed with each BMI category. Compared with the children of normal-weight mothers, death from any cause was 11% more frequent among the offspring of mothers who were overweight, 27% more frequent with grade I obesity, 68% with grade II and 92% with grade III. When BMI was treated as a continuous scale, the authors found that the curve was relatively flat at lower BMI levels and began to rise from a BMI of about 23, which is inside the normal range.

The causes of death were mixed. The link appeared for deaths from birth defects, lung disease and cardiovascular disease, and also for what the registers class as unnatural deaths, meaning accidents and suicide.

What cousins reveal about maternal BMI and family background

Families differ in ways no register captures: in genes, in what is eaten at home, in money and in neighborhoods. Weight runs in families and so does poor health, so a link between a mother’s BMI and her child’s survival could reflect the family and have little to do with the pregnancy.

To probe this, the team compared cousins. Cousins share grandparents and much of an extended family’s circumstances, but they have different mothers, who may have had different weights. The analysis identified 980,355 individuals nested within 302,446 extended families.

Among cousins the associations weakened. For maternal overweight and grade I obesity, the difference in death rates was no longer distinguishable from chance. For grades II and III it remained, at 58% and 68% higher. The authors summarize that the results from the cousin comparison analysis were directionally consistent with the main analysis but smaller. Shared family background seems to account for part of the link, and not all of it at the highest weights.

Why might a mother’s weight be tied to offspring mortality?

The most direct route runs through pregnancy itself. Obesity raises the chance of preterm birth, high blood pressure in pregnancy and gestational diabetes, each of which carries risks for the baby. The team calculated how much of the association passed through such complications and report that the estimated proportion mediated by these perinatal factors was 25.8% for maternal obesity, where perinatal means around the time of birth. Three quarters of the link lay elsewhere.

Beyond that the paper lists hypotheses. Conditions in the womb may shape how organs develop, an idea known as the developmental origins of health and disease. Obesity is accompanied by low-grade inflammation and altered metabolism that could affect a growing fetus. Experimental studies point to changes in how genes are switched on and off.

The authors then set limits on their own speculation. They write that “these potential biological mechanisms should not be interpreted as evidence of a direct causal effect of maternal BMI on offspring mortality”, and that maternal BMI may also function as a marker of a broader set of health and social conditions that cluster within families. The accident and suicide deaths illustrate the point. There is no obvious biological path from weight in pregnancy to a road crash twenty years later, and the authors say these associations may be more likely to reflect residual confounding or unmeasured familial and social factors. Residual confounding is the influence of things a study did not or could not adjust for.

Earlier studies of maternal obesity stopped at infancy

The first year of life is well mapped. A 2014 study in The BMJ by Stefan Johansson and colleagues, also at Karolinska, examined 1,857,822 live single births in Sweden and found that infant deaths rose from 2.4 per thousand among normal-weight mothers to 5.8 per thousand among those with the most severe obesity. In the same year Dagfinn Aune of Imperial College London pooled 38 studies in JAMA. That team estimated that for mothers with a BMI of 20, 25 and 30, there were 33, 37 and 43 infant deaths for every 10,000 pregnancies, and concluded that even modest increases in maternal BMI went with higher risk.

A 2024 analysis of almost 19 million American births added sudden unexpected infant death to the list, with a risk that increased with increasing obesity severity.

Past infancy there was a single precedent. Rebecca Reynolds of the University of Edinburgh traced 37,709 people born in Aberdeen from 1950 onward through to deaths and hospital admissions in offspring aged 34-61. All cause mortality was increased in offspring of obese mothers, by about a third. That study was too small to look at causes of death or at the years in between. The Swedish cohort is seventy times larger, covers the whole span from birth, and is the first to use a family comparison.

Gaps in the maternal BMI data: fathers, habits and the children’s own weight

The study knew a great deal about mothers and almost nothing about the children as they grew. The authors note that they lacked complete and harmonized information on offspring characteristics, such as BMI, socioeconomic status, and lifestyle factors. Children of heavier mothers are more often heavy themselves, and may smoke, eat and exercise differently, which could carry much of the adult risk without any involvement of the womb.

The register held no weight for fathers. Studies of this kind sometimes use the father’s BMI as a check, because a similar pattern for fathers would point toward shared family environment more than toward the biology of pregnancy.

The cousin design has its own blind spot. In the authors’ words, it does not account for non-shared factors, such as individual dietary patterns, sedentary behaviors, or alcohol use. Mothers within one extended family differ in their habits as well as their weight.

Finally, weight was missing for some births. Maternal BMI during early pregnancy was available for 83% of all women, with the gaps concentrated in the earliest years, although restricting the analysis to births from 1992 onward gave similar results.

Does maternal BMI matter for an individual family?

For an individual family, very little. The absolute risks are small: in a country where fewer than one person in a hundred dies before their forties, even the largest relative increase in this study leaves the great majority unaffected. The study did not test whether losing weight before pregnancy changes how long children live.

The authors pitch their conclusion at that level of caution. If confirmed by further studies, they write, the results highlight the importance of achieving a healthy maternal BMI prior to conception as a potential strategy to improve child health. The word carrying the weight in that sentence is “potential.”

A mother’s weight in early pregnancy tracks with her children’s survival for decades in these records, and how much of that reflects the pregnancy and how much the family is still unknown.