A Mother's Weight Early in Pregnancy and Her Child's Health Over a Lifetime

What a study of 2.6 million Swedish births (Wang et al., PLoS Medicine 2026) tells families about weight, newborn risk and the limits of the evidence

A study that followed 2.6 million children born in Sweden found that babies whose mothers had a higher weight at the start of pregnancy were somewhat more likely to die young, especially in their first year, although the great majority of all children did well. The link was clearest for the highest weight levels, and the study cannot show that a mother's weight itself caused the extra risk.

Many families in neonatal units have questions about whether something about the pregnancy affected their baby. Weight is one of the hardest of these questions to talk about, because it touches on health, body image and feelings of blame. This article explains what a large new study found, what it does not show, and why researchers are careful not to turn a statistical link into a judgement about any individual mother.

Why researchers have been asking about weight and pregnancy

Doctors usually describe weight using the body mass index, or BMI, which compares a person's weight with their height. A BMI between 18.5 and 24.9 is called the "normal" range; 25 to 29.9 is "overweight"; and 30 or above is "obesity", which is split into three grades, with grade III meaning a BMI of 40 or more. These labels are rough tools for comparing groups of people, not verdicts on any one person's health.

Over recent decades, more people have been starting pregnancy with a higher BMI. In the Swedish study described here, the share of mothers in the overweight range rose from about 1 in 10 in 1982 to about 1 in 4 in 2014, and the share living with obesity rose from 2.0% to 12.9% [1]. Around the world, roughly one pregnancy in six now involves maternal obesity [2], and among women obesity has become more common than being underweight in most countries [3].

Researchers have long known that a higher weight before pregnancy is linked with more pregnancy complications, such as high blood pressure, diabetes that starts during pregnancy, and babies who are born larger than expected [4]. Over time, studies began to look at the babies themselves. A Swedish study of nearly 1.9 million births found that about 2.4 in every 1,000 babies of mothers in the normal weight range died before their first birthday, compared with 5.8 in every 1,000 when mothers had the highest grade of obesity [5]. A review that combined 38 studies found that even modest increases in a mother's BMI were linked with a higher risk of a baby dying in infancy [6], and a large American study linked maternal obesity with sudden unexpected infant death, the term doctors use when a baby dies suddenly and without warning [7]. Experts also suspected that effects could last into later life [8], but according to the new study's authors, only one earlier study, from Scotland, had followed children into adulthood to look at deaths, and it was too small to answer detailed questions [1][9]. Nobody had yet checked whether the link held up when children were compared with close relatives who share much of the same family background [1].

How the Swedish study was done

Sweden keeps very complete national health records. Since 1982, the country's birth register has recorded mothers' measured weight and reported height during pregnancy [10], and a separate national register has recorded virtually all deaths for decades [11]. The research team, from Karolinska Institutet, Uppsala University and Shanghai Jiao Tong University School of Medicine, linked these records for 2,606,684 babies born alive in Sweden between 1982 and 2014 [1]. (Births in 1990 and 1991 were left out because mothers' weight was not recorded in those years.)

Each mother's BMI came from her first pregnancy check-up, which in Sweden usually happens within the first 14 weeks. The children were then followed for typically about 22 years, and some for up to 42 years. This is an observational study: nobody was asked to change anything, and researchers simply looked at what happened. To make the comparisons fairer, they took into account many other factors that might matter, such as the mother's age, education, income, smoking and earlier health conditions.

They also did something unusual. They compared children with their cousins whose mothers were sisters. Cousins share part of their family background, genes and upbringing, so if the link disappeared within families, it would suggest that family circumstances, rather than weight, were responsible. Comparisons within families like this are a recognised way of testing explanations in research [12].

What the researchers found

Out of more than 2.6 million children, 21,981 (less than 1 in 100) died during the years of follow-up. More than a third of these deaths, 8,135, happened in the first year of life [1].

The higher the mother's BMI, the higher the child's risk tended to be. Compared with children of mothers in the normal range, the risk of death over the whole follow-up was about 11% higher with maternal overweight, 27% higher with obesity grade I, 68% higher with grade II and 92% higher, close to double, with grade III. Being underweight was not linked with higher risk [1].

Percentages like these can sound frightening, so it helps to look at real numbers. In the first year of life, about 2.9 in every 1,000 babies of mothers in the normal weight range died. That figure was about 3.3 in 1,000 with maternal overweight, 3.6 with obesity grade I, 5.2 with grade II and 5.8 with grade III. Put another way, even in the highest weight group, more than 99 out of every 100 babies lived through their first year. These simple counts, worked out from the study's published numbers, do not adjust for other factors [1].

The link appeared both in the first four weeks after birth and later in the first year. It was not seen between ages 1 and 5, and showed up again, less consistently, in the teenage years and adulthood. In babies, the specific causes of death linked to a mother's higher weight included breathing problems and birth defects. Later in life, heart and blood vessel disease and cancer played a larger part [1].

What the cousin comparison and other checks showed

When children were compared with their cousins, the links became weaker. For mothers in the overweight range and for obesity grade I, the difference could no longer be clearly separated from chance. For obesity grades II and III, the higher risk remained [1]. This suggests that, at more modest weights, shared family circumstances may explain much of the link, while at the highest weights something more may be going on.

The researchers also asked whether pregnancy complications explained the link. Being born too early is linked with a higher risk of death well into adulthood [13], and it accounted for about 16% of the link with maternal obesity. All the measured complications together accounted for about a quarter. Even when every pregnancy with a measured complication was set aside, the pattern remained [1]. So most of the link is not explained by the problems doctors already watch for, and scientists do not yet know why.

What this means for families

The authors are clear that their study does not prove a mother's weight causes these deaths. Weight is tied up with many other parts of life, including income, education, diet, other health conditions and family circumstances, and the researchers could not measure all of them [1]. A mother's weight is never the only story, and no parent should read these results as blame for their child's illness. Most children of mothers with a higher BMI are healthy and grow up well.

What the study does suggest is that a mother's weight early in pregnancy is one useful piece of information for doctors and nurses, alongside things like how early a baby was born. For families with a newborn, it is a good reason to ask questions, keep up with routine check-ups and follow safe-sleep advice, which matters for every baby. For people thinking about a future pregnancy, it may be worth talking with a doctor or midwife before conception about overall health, including weight, in a supportive way. Scientists have suggested that the time around conception may shape a child's later health [14], and that changes in metabolism during pregnancy may affect how the placenta and baby develop [15], but these ideas still need testing.

What researchers are working on next

The key question is whether helping people reach a healthier weight before pregnancy would actually improve children's health and survival. That can only be answered by studies that offer support and compare outcomes, which the study's authors say are still needed [1]. Researchers also want to understand why babies of mothers with the highest BMI had more deaths after the first month, and whether the findings hold in countries where pregnancy care and child health differ from Sweden.

This study adds careful, very large-scale evidence to an important conversation. It shows a real but modest link between a mother's weight early in pregnancy and her child's chance of dying young, especially in the first year, while reminding us that the link is not proof of cause and that the overwhelming majority of babies, whatever their mother's weight, survive and thrive.

References

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  2. Kent L, McGirr M, Eastwood KA. Global trends in prevalence of maternal overweight and obesity: A systematic review and meta-analysis of routinely collected data retrospective cohorts. Int J Popul Data Sci. 2024;9(2):2401. doi:10.23889/ijpds.v9i2.2401 ↩
  3. NCD Risk Factor Collaboration (NCD-RisC). Worldwide trends in underweight and obesity from 1990 to 2022: a pooled analysis of 3663 population-representative studies with 222 million children, adolescents, and adults. Lancet. 2024;403(10431):1027-1050. doi:10.1016/S0140-6736(23)02750-202750-2) ↩
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