ObjectiveTo evaluate the fetal iliac wing angle in detecting trisomy 21 in the second trimester of pregnancy.MethodsUsing an axial view of the fetal pelvis, the angle between the right and left iliac wings (iliac wing angle) was measured ultrasonographically at the time of the secondtrimester ultrasound or genetic amniocentesis in 377 singleton fetuses. Trisomy 21 was diagnosed by karyotype results from the amniocentesis or newborn examination with karyotype if trisomy 21 was suspected based on phenotypic features. Sensitivity, specificity, and positive (PPV) and negative predictive values (NPV) were calculated using multiple cutoff points. A receiver operating characteristic (ROC) curve was used to identify the optimum iliac wing angle. Descriptive statistics and Student t test were utilized for analyses with aPof less than .05 considered significant.ResultsThe average gestational age was 18.8 weeks (range 13–32). Karyotypes were available in 128 fetuses. The overall prevalence of trisomy 21 was 11 of 377 (2.9%). The mean (± standard deviation) iliac wing angle in the normal fetuses was 68.2 degrees (±15.4 degrees) and 98.5 degrees (±11.3 degrees) in fetuses with trisomy 21 (P< .001). Using an ROC-derived absolute cutoff of 90 degrees, an abnormal iliac wing angle had sensitivity of 90.9% (ten of 11), specificity of 94.5% (346 of 366), NPV of 99.7% (346 of 347), and PPV of 33.3% (ten of 30) to detect trisomy 21.ConclusionFetuses with trisomy 21 have greater iliac wing angles than do normal fetuses. Using an ROC-derived absolute cutoff of 90 degrees, we could detect 90.9% of fetuses with trisomy 21 with a PPV of 33% in our high-risk population. These findings suggest that iliac wing angle is a useful marker in antenatal screening for trisomy 21.