(CNN) - Pregnant women who are obese or overweight have an increased risk of delivering a stillborn baby, according to a new study published Tuesday in the Journal of the American Medical Association.
Researchers looked at 38 studies to better understand the potential risks to an unborn child in relation to its mother's body mass index. They found even a modest increase in an obese pregnant woman's weight is linked to an increased risk of fetal death, stillbirth and infant death.
The highest risk was in women with a BMI over 40 (30 is considered obese). These women were two to three times more likely to experience complications. Even women with a BMI over 25 (which is considered overweight) were found to experience increased complications.
While the risk is increased for women with higher BMIs, the absolute risk is low, meaning it's uncommon.
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.
The study findings were not a surprise, Conry said, obesity is already known to be associated with complications that can lead to an unhealthy fetus.
Conry suggests women engage in "preconception planning" and decide what their reproductive goals are for the year to ensure a healthy mom and baby. She recommends women who aren't ready to have a baby use contraception.
And when a woman decides she wants to have a baby, Aune said, she "needs to be close to her ideal body weight, needs to be exercising, needs to have blood pressure and blood sugar in good control, and should not be on any medications that can cause birth defects."
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.