If pregnant women are obese, the ability of ultrasound to identify major fetal abnormalities is reduced, according to researchers at the University of Texas Southwestern Medical Center in Dallas.

This finding stems from a study of 11,135 pregnancies in which sonogram examinations were performed at 18 to 24 weeks. The mothers’ body mass index - a measure of weight in relation to height - at the first prenatal visit was classified as normal in 39 percent of mothers, overweight in 34 percent, and obese in 27 percent.

There were 181 infants born with abnormalities, Dr. Jodi S. Dashe and co-researchers report in the journal Obstetrics & Gynecology. Standard ultrasound detected 53 percent of the abnormalities at the fetal stage, and a more targeted ultrasound examination by doctors with expertise in prenatal diagnosis identified 68 percent.

Among patients who underwent standard ultrasound, the ability to detect abnormalities decreased as body weight rose. For instance, in normal weight women, 66 percent of abnormalities were identified, whereas in severely obese women, the detection rate fell to just 25 percent.

Among those who had targeted ultrasound exams, by contrast, no weight-based differences in detection rates were noted.

Fetal abnormalities were often missed in diabetic women. The authors believe that this relates to the fact that diabetic women often have increased obesity in the abdominal region where ultrasound is performed.

“Counseling may need to be modified to reflect the limitations of ultrasonography in obese women,” they conclude.

SOURCE: Obstetrics & Gynecology, May 2009.