WEDNESDAY, April 22 (HealthDay News) -- Overweight and obese women are less likely to receive an accurate reading from ultrasound screenings aimed at identifying fetal abnormalities, new research reveals.

Such screenings also seem to be less effective among women who are diabetic prior to their pregnancy.

"We were asking the question, 'Are birth defects less likely to be detectable with prenatal ultrasound in women who are overweight or obese, compared with women of normal body-mass index,'" explained study author Dr. Jodi S. Dashe, who works in the department of obstetrics & gynecology at the University of Texas Southwestern Medical Center in Dallas.

After analyzing more than 10,000 standard ultrasound exams -- the most common exam for low-risk pregnancies -- as well as more than 1,000 specialized ultrasound exams for high-risk pregnancies, Dashe found, "that the detection of fetuses with major birth defects dropped significantly as maternal body-mass index [BMI] increased: a difference of at least 20 percent when women of normal body-mass index were compared with obese women."

The findings are to be published in the May issue of Obstetrics & Gynecology.

To explore the degree to which ultrasound results are sensitive to BMI, the study authors looked at a total of 12,200 ultrasounds previously performed at the medical center between 2003 and 2007, among women entering their 18th to 24th week of pregnancy.

The majority of both standard-risk and high-risk pregnant women were Hispanic (about 87 percent), with about 10 percent participants being black and 2 percent being white.

Stacking ultrasound results up against both the mother's BMI and infant discharge records, the research team noted the 20 percent weight-linked plunge in ultrasound effectiveness. That translated into a significant rise in risk that a child would be born with a major birth defect despite the fetus having appeared normal during an ultrasound.

For example, the specific risk for incorrectly receiving a "normal" ultrasound reading rose from just one in every 250 women with normal BMI to one among every 100 obese women.

Apart from the BMI effect, the authors also found that among cases deemed to face a particularly high risk for fetal abnormalities prior to undergoing a screening, ultrasound mistakes were more likely to occur among women who had been diabetic prior to their pregnancy than among women with other high-risk indications.

In fact, diabetic women were found to experience only a 38 percent success rate in spotting fetal abnormalities via ultrasound, compared to the 88 percent success rate experienced among women burdened with other sorts of high-risk factors.

One expert said that patient obesity is only one of the challenges obstetricians deal with when reading an ultrasound.

"It isn't that ultrasounds are unreliable, but that with some types of patients it is certainly more difficult to visualize fetal anomalies," said Dr. Catherine Y. Spong, chief of the pregnancy and perinatology branch of the Eunice Kennedy Shriver National Institute of Child Health and Human Development in Bethesda, Md.

"When I'm doing an ultrasound I'm trying to get as much information as I can about the baby," she explained. "And some people are easier to do an ultrasound on than others. For example, if someone has a lot of scarring inside the tissue is thicker, and that can make the imaging more difficult. And if people are obese -- including women with diabetes, who are not uncommonly overweight -- you're going to have to look through a lot more tissue, and that makes it more difficult to image."

Dashe and her colleagues noted that American women with normal BMI now constitute a minority of the country's pregnant population, so these screening problems could have "broad implications."

"Based on our findings, we would suggest that counseling be modified to reflect the limitations of ultrasound in women who are overweight or obese," she said. The researchers also advise that more research be conducted to examine the particular risk for screening problems observed among diabetic women.

Spong agreed. "This difficulty presents itself while there has been a dramatic increase across the U.S. in terms of obesity and morbid obesity among reproductive-age women," she said. "So this study provides physicians with some data on what the difficulty truly is."

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• Obesity
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