Maternal weight may affect the ability of ultrasound to detect fetal abnormalities during the second trimester, according to research published in the May Obstetrics and Gynecology.

Jodi S. Dashe, MD, of the University of Texas Southwestern Medical Center in Dallas, and colleagues analyzed data from ultrasound examinations given during pregnancies of 18 to 24 weeks over a 5-year period. These included 10,112 standard examinations and 1,098 targeted examinations. The detection of an anomalous fetus was defined as the identification of a major abnormality of a relevant organ system, and anomalies were verified using a database of birth defects.

As body mass index (BMI) rose, the detection of anomalous fetuses decreased. For normal BMI, detection with standard and targeted ultrasonography was 66% and 97%, respectively.

In the highest class of obesity, these values fell to 25% and 75%, respectively. The residual anomaly risk─delivering an infant with major anomaly after no abnormality was found on ultrasound─rose from 0.4% in women with normal BMI to 1.0% in obese women.

“Based on our findings, we suggest that when performing ultrasonography in overweight or obese pregnancies, counseling may need to be modified to reflect the limitations of both standard and targeted ultrasonography. For example, after a normal ultrasound evaluation, obese women without diabetes may still have up to 1% risk of a major anomaly─including some anomalies not detectable ultrasonographically,” the authors write.

Dashe JS, McIntire DD, Twickler D. Effect of maternal obesity on the ultrasound detection of anomalous fetuses. Obstet Gynecol. 2009;113:1001-1007 (doi: 10.1097/AOG.0b013e3181a1d2f5)

Contemporary OB/GYN