People born prematurely are likely to have similar body mass indexes (BMIs) when they reach adolescence to people who were born at term.

To determine what effect the amount of time spent in the uterus has on a person’s body weight in childhood and adolescence, Johan Vinther at the University of Copenhagen in Denmark and his colleagues analysed data collected across 16 studies conducted in Europe, North America and Australia.

These studies included data on the gestation age of newborn children and follow-up data on their subsequent heights and weights over several years. A total of more than 250,000 children were included from across the studies, all born between 1985 and 2017.

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Vinther and his colleagues took the height and weight data from each person and calculated their BMIs at various ages.

“Body size is a decent proxy for physical health,” says Vinther. “Though it’s not the only measure.”

The researchers found that while people born before 37 weeks of pregnancy, also known as preterm, were more likely to have a lower BMI in childhood than people born after 37 weeks, this difference disappeared between the ages of 14 and 19.

“Our study suggests that preterm individuals gain more weight in childhood, relatively, compared to people born at term,” says Vinther. It is unclear why, he adds.

The findings don’t necessarily mean children born preterm will always reach healthy body weights in adolescence, says Vinther. Other factors such as the BMI and education level of the mother will play a role, he says.

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The results are based on people in high-income countries. “We don’t really know what this would look like in lower-income countries,” says Vinther. The team didn’t look at the specific effect of sex or ethnicity on the results.

Neena Modi at Imperial College London says the findings corroborate several long-running studies on this issue from across the world. However, recent work has shown that very preterm individuals are more likely to develop chronic conditions that are exacerbated by being overweight, she says.

“The critical health message is to ensure preterm individuals, and their parents and clinicians, are aware of the importance of maintaining a healthy body weight,” she says.

PLoS Medicine DOI: 10.1371/journal.pmed.1004036

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