Healthy-weight women who gained weight between their first and second pregnancies—even as little as 12 pounds for someone of average height—had an increased risk of a stillbirth or their baby dying within the first year of life, compared with women whose weight remained stable, according to a Swedish study.

The new study, based on a database of more than 456,000 Swedish women and published in the U.K. medical journal the Lancet, is thought to be the first to examine how weight gain between pregnancies is connected to risk of infant death. It suggests that weight gain during this period may be a modifiable risk for reducing stillbirth and infant mortality, which researchers say hasn’t declined much in recent decades in many high-income countries.

The risk of stillbirth and infant mortality remained small overall, but the relative risk was markedly higher for women who gained weight between pregnancies. For women who kept their weight steady, the infant mortality rate was two per 1,000. That rose to 2.3 per 1,000 for women who started off at a normal weight and put on about 12 pounds (5.6 kilograms). The rate of stillbirths increased to three per 1000 from two per 1,000. For women who put on about 12 pounds, the infant mortality risk increased by 30%. The risk increased further with more pronounced weight gain.

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Women who were already overweight, whose babies are known to have an elevated risk of both outcomes to begin, didn’t have a greater risk with respect to infant mortality. But overweight women who lost weight between pregnancies reduced the risk of neonatal mortality—their baby dying in the first 28 days of life—for their second babies, according to the study.

“From a public-health perspective, it is possible that lives could be saved if we lived in the ideal world and you go back to your ideal weight,” said Sven Cnattingius, a professor in reproductive epidemiology at the Karolinska Institute in Sweden and first author on the study.

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