The danger is greatest for severely obese women, who appear to have about double or triple the risk of losing their baby, although that risk is still small, the study authors noted.
The findings, based on a review of previously published studies, underscore the need for women who plan pregnancy to try to maintain a healthy weight, the researchers suggested.
"As for women who are already pregnant, they should follow existing guidelines for weight gain during pregnancy," said lead author Dagfinn Aune, from the department of epidemiology and biostatistics in the School of Public Health at Imperial College London in England.
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Obesity and type 2 diabetes
"This analysis gives a better picture of the strength of the risks," said Aune. "Although foetal and infant deaths are relatively rare in high-income countries, affecting about 0.5% of pregnancies, they are devastating for the parents that are affected."
Moreover, overwhelming data shows that being overweight or obese increases the risk for type 2 diabetes, gestational diabetes, pre-eclampsia, high blood pressure and birth defects, Aune said.
"All of these conditions have been linked to increased stillbirth risk. Although we don't know all the details of the molecular mechanisms, I think it's likely that there is a biological effect of excess weight on these outcomes," he said.
For the study, Aune's team analysed 38 previously published studies that looked at the association between weight before and during early pregnancy, and death of the foetus or infant.
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Body mass index
These studies included more than 10 000 miscarriages, over 16,200 stillbirths and more than 4 300 deaths near the time of birth. Nearly 11 300 deaths during the first month of life and just under 5000 infant deaths later were also covered.
Overweight and obesity is determined by body mass index (BMI), a calculation based on height and weight. A woman 5 feet 2 inches tall who weighs 220 pounds has a BMI of 40, which is considered severely obese.
A woman of the same height who weighs 105 to 130 pounds would have a BMI between 19 and 24, which is considered normal weight. At 130 to 160 pounds, her BMI would be 25 to 29, which is considered overweight.
Even modest increases in the mother's weight was linked to an increased risk of infant death, the researchers found.
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