Objective: Pelvic bone abnormalities are common in newborns with Down syndrome. The aim of this study was to evaluate the utility of ultrasonographic iliac bone length measurement in the identification of fetuses with Down syndrome in the second trimester of gestation.
Study design: Ultrasonographic iliac length measurement was obtained, in an axial view of the fetal pelvis, on 10 fetuses with Down syndrome and 180 karyotypically normal fetuses before genetic amniocentesis. Regression equations relating biparietal diameter to iliac length measurement were used to calculate ratios of observed-to-expected length, sensitivity, and specificity at various cutoff points.
Results: Linear regression of iliac length measurement on biparietal diameter for normal fetuses resulted in the following equation: iliac length measurement (centimeters) = -0.2723 + 0.0333 biparietal diameter (millimeters). Iliac length measurement in Down syndrome fetuses was significantly longer than in normal controls (p < 0.0001). A ratio of 1.21 for observed-to-expected iliac length measurement yielded a sensitivity of 40%, a specificity of 98%, and positive predictive values of 50% and 2.60%, respectively, in populations at risk for Down syndrome of 1 in 20 and 1 in 750.
Conclusion: Iliac length measurement is increased in fetuses with Down syndrome. An observed-to-expected iliac length measurement of > or = 1.21 has a positive predictive value of 1/38 in a low-risk population with a false-positive rate of 2%. This preliminary study suggests that iliac length measurement may be useful as an ancillary screening variable in antenatal screening for Down syndrome.