Researchers looked at 38 studies to better understand the potential risks to an unborn child in relation to its mother's body mass index.
"We found weak, but statistically significant increases in risk even within what is considered the normal ranges of BMI," lead study author Dagfinn Aune said in an e-mail. "For example, the relative risk of stillbirth, perinatal and infant death increased by 20%, 11% and 10% already at a BMI of 25 compared with a BMI of 20, however, the really strong associations were observed among women who were either obese or severely obese who had up to two- to three-fold increases in the risk compared to lean women."
But "it is quite devastating for the parents it happens to," he said. "I think at least when you get into the obese or severely obese range there is reason for concern. In addition, overweight and obesity is associated with increased risk of many other pregnancy complications, so when you look at the total picture it's not trivial."
Women need to be at their optimum health before deciding to have a baby, said Dr. Jeanne A. Conry, president of the American College of Obstetricians and Gynecologists.
"Healthy lifestyle is critical. Eating healthy and getting 30 minutes of exercise every day are among the most important choices a woman can make. Fifty percent of pregnancies are unplanned, so a woman who is not fit and not using contraception places her infant and herself at risk," Conry said.
"A higher risk of birth defects is seen in women who are obese, and some of these defects may not be compatible with life, or may result in an infant that is compromised," she said. "Obese women have a much higher incidence of diabetes and of hypertension, and such medical problems may lead to early delivery that results in the loss of an infant."
Biological mechanisms could help explain the association between increasing maternal BMI and risk of fetal and infant death, the study suggests.
Higher BMIs are also associated with increased risk of other pregnancy complications, Aune said, such as "gestational diabetes, preeclampsia, gestational hypertension, macrosomia (oversized fetus) and congenital anomalies." Those can all lead to a higher risk of stillbirths or infant deaths.
"One study has suggested that obstetric conditions and placental abnormalities may explain approximately half of all stillbirths," Aune said. "It has also been suggested that thinner women may be better at recognizing decreased fetal movements, which may precede fetal deaths."