Women who are carrying extra weight before pregnancy or early in gestation are at heightened risk of having their infants die shortly before or after birth, according to a new analysis of past research.
The risk was greatest among the most obese women, the authors write in The Journal of the American Medical Association.
"The main message of the study is that maternal overweight and obesity increases the risk of fetal death, stillbirth and infant death," said Dagfinn Aune, the study's lead author, from Imperial College London.
"Since excess weight is a potentially modifiable risk factor, further studies should assess whether lifestyle and weight changes modifies the risk of fetal and infant death," he told Reuters Health in an email.
Stillbirths, when a child dies in the womb toward the end of pregnancy, account for a large part of the estimated 3.6 million neonatal deaths that occur each year, the researchers point out.
Previous studies have linked women's weight during pregnancy to the risk of their children dying in the womb or shortly after delivery due to complications. Some could not show their findings were not due to chance, however.
For the new study, the researchers pulled together data from 38 studies. Together, these included over 45,000 accounts of child deaths that occurred shortly before or after delivery, although a few studies counted deaths up to one year after birth.
According to the U.S. National Institutes of Health, a person of normal weight would have a body mass index (BMI) - which is a measure of weight in relation to height - between 18.5 and 24.9.
An adult who is 120 pounds and five feet, five inches tall, for example, would have a BMI of 20.
A BMI between 25 and 29.9 is considered overweight, and a score of 30 or above is considered obese. (See BMI calculator, here: 1.usa.gov/MC4ERL).
The researchers found there were about 76 stillbirths per 10,000 pregnancies among women of normal weight. That increased to 82 stillbirths for women with a BMI of 25, and 102 among women with a BMI of 30.
Similar increases in risk were observed for other categories of infant death.
Children of women in the severely obese BMI category of 40 or above had a rate of infant death about two to three times higher than women with a BMI of 20.
"There was about a 20 percent increased risk of loss for every 5 BMI points that women's weight increased," Dr. Christopher Glantz, a high-risk pregnancy expert who was not involved in the new study, told Reuters Health.
"I think the interesting thing about it is that they got somewhat similar results no matter what way they looked at it," said Glantz, of the University of Rochester in New York.
The researchers write that the increased risk of death might be explained by an increased risk of complications among overweight and obese mothers.
For heavier women who are planning to get pregnant, Glantz said it would be ideal to lower their body weight.
"That would be our dream," he said. "The difficult problem we have is, once we see these patients, they're already pregnant."
Aune said that women who are already overweight during their pregnancy should seek help from their doctors, who will give them advice with regard to the optimal weight gain to prevent pregnancy complications.
"I think it makes sense to recommend obese pregnant women to be active as it will help control the weight gain and reduce the risk of these other pregnancy complications," he said, adding that pregnant women are still not advised to lose weight.
Aune noted that his team recently found that physical activity before and during pregnancy may reduce the risk of preeclampsia.
"So one next step which we hope to investigate is whether physical activity might reduce the risk of stillbirths as well," he added.