Effect of maternal obesity on the ultrasound detection of anomalous fetuses

To estimate the effect of maternal habitus on detection of fetuses with major structural anomalies during second-trimester standard and targeted ultrasound examinations.

This was a retrospective cohort study of pregnancies 18 to 24 weeks that underwent ultrasonography over a 5-year period. An anomalous fetus was considered detected if a major abnormality of the relevant organ system was identified, regardless of the anticipated ultrasound detection. Anomalies were verified using a prospectively maintained database. Body mass index (BMI) was based on weight at first prenatal visit.

There were 10,112 standard examinations in low-risk pregnancies and 1,098 targeted examinations in pregnancies with either high-risk indications or with an abnormality detected during standard ultrasonography. Detection of anomalous fetuses decreased with increasing BMI. For normal BMI, overweight, and class I, II, and III obesity, detection with standard ultrasonography was 66%, 49%, 48%, 42%, and 25%, respectively, and with targeted ultrasonography, 97%, 91%, 75%, 88%, and 75%, respectively, both P< or =.03. Residual anomaly risk after a normal ultrasound examination increased with increasing BMI, from 0.4% among women of normal BMI to 1.0% among obese women, P=.001. Anomaly detection was lower among women with pregestational diabetes than in those with other high-risk indications, 38% compared with 88% respectively, P<.001.

With increasing maternal BMI, we found decreased detection of anomalous fetuses with either standard or targeted ultrasonography, a difference of at least 20% when women of normal BMI were compared with obese women. Anomaly detection was even less in pregnancies complicated by pregestational diabetes. Counseling may need to be modified to reflect the limitations of ultrasonography in obese women.