Birth weight prediction method may lead to safer deliveries

Physicians at the University of Rochester Medical Center have identified a method that appears to reliably predict the birth weight of babies in obese pregnant women, a key factor in counseling patients and planning for safe delivery of their babies.

The study, published in the July issue of Ultrasound in Obstetrics and Gynecology, used a method called gestation-adjusted projection (GAP) to accurately predict birth weight in more than nine out of 10 cases, according to lead author Loralei Thornburg, M.D., a maternal-fetal medicine specialist at the Medical Center.

"Obesity is a risk factor for almost all obstetric complications," said Thornburg. "This method can help us more accurately predict macrosomia - high birth-weight babies over 4,000 grams or about nine pounds - which is associated with complications and higher death rates for mothers and babies. Given that two-thirds of Americans are obese and one in 20 is morbidly obese, it is essential that clinicians are able to predict which mothers will give birth to high birth-weight babies, and to be able to reassure those who are not carrying one larger than average."

The GAP method uses ultrasound images taken between the 34th and 36th week of pregnancy, rather than at the end of pregnancy when it is more difficult to visualize the baby, Thornburg said. These ultrasound measurements, coupled with a mathematical formula, give doctors a sense of how the baby is growing so a birth weight prediction can be made. This is especially helpful in obese pregnant woman, in whom visual ultrasound is less accurate and for whom the risk for obstetric complications is higher.

The study reviewed records of nearly 1,400 women who had fetal ultrasound exams when they were between 34 and 36 weeks of pregnancy. Researchers used patients' body mass index (BMI) to divide them into categories, with the obese patients separated in to three groups: BMI 30 to 35, BMI 35 to 40, and BMI 40 or greater.

Overall, the GAP method was able to predict the birth weight within 20 percent in 93 to 95 percent of cases, within 15 in 82 to 86 percent of cases, and within 10 percent in 59 to 70 percent of cases.

"We found that the GAP method performed equally well for the obese and normal-weight pregnant women in our study, with some decrease in accuracy in the most obese women. Most importantly, the method allowed us to rule out the risk of macrosomia in more 80 percent of cases, regardless of the mother's BMI," Thornburg said. "Obesity and high-weight babies pose extra risks for pregnant women and present real and growing challenges for health professionals. Being able to identify those risks more clearly is an important step in caring for obese pregnant women and their babies," Thornburg said.

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