Women with a higher body mass index (BMI) before or in early pregnancy may have a risk of fetal death, stillbirth and infant death, British researchers said Tuesday.

The risk is highest for women who are severely obese, the researchers reported in the U.S. journal JAMA.

There are about 2.7 million stillbirths in 2008 and about 3.6 million more babies die within the first 28 days of life each year.

Several studies have suggested that greater BMI before or during early pregnancy is associated with an increased risk of fetal death, stillbirth, perinatal death (stillbirth and early neonatal death), neonatal death, and infant death.

BMI is a measure of body fat based on weight and height. A BMI between 19 and 24 is considered normal and healthy, while a BMI between 25 and 29 is overweight, and a BMI over 30 is considered obese. An optimal pregnancy BMI to prevent fetal and infant death, however, has not been established.

Dagfinn Aune of Imperial College London and colleagues conducted a review to examine the association between maternal BMI and risk of fetal death, stillbirth, and infant death.

After a search of the medical literature, the researchers identified 38 studies that included more than 10,147 fetal deaths, more than 16,274 stillbirths, more than 4,311 perinatal deaths, 11, 294 neonatal deaths, and 4,983 infant deaths.

The researchers found that even modest increases in maternal BMI were associated with increased risk of fetal death, stillbirth, neonatal death, perinatal death, and infant death.

The greatest risk was observed in the category of severely obese women. Women with a BMI of 40 had an approximate two to three-fold increase in these risks compared with women with a BMI of 20.

The researchers said conditions related to overweight and obesity, including preeclampsia, gestational diabetes, type 2 diabetes, gestational hypertension and congenital anomalies, may be to blame for the increased risk of fetal and infant death.

"Weight management guidelines for women who plan pregnancies should take these findings into consideration to reduce the burden of fetal deaths, stillbirths, and infant deaths," the researchers added.