The iliac angle as a sonographic marker for Down syndrome in second-trimester fetuses

To determine whether measurements of the iliac wing angle on sonograms of the second-trimester fetus can be used to detect fetuses at increased risk for Down syndrome.

All second-trimester Down syndrome fetuses karyotyped by amniocentesis over 5 years for whom adequate views of the iliac bones were available were reviewed retrospectively. The control group consisted of consecutive, normal second-trimester fetuses undergoing karyotyping during the same period. The angles between the iliac bones of the fetal pelves were measured on transverse sonograms, and the two groups were compared. A separate prospective evaluation of the ideal level for measurement of the iliac angle was made in two fetuses, one with and one without Down syndrome.

We evaluated 38 Down syndrome and 46 control fetuses. The mean iliac-angle measurements differed significantly for the Down syndrome fetuses as compared with normal control fetuses: 78.8 degrees +/- standard deviation (SD) 18.5 and 66.9 degrees +/- SD 13.9, respectively. An iliac angle of at least 90 degrees was measured in 14 (36.8%) Down syndrome fetuses and in two (4.3%) control fetuses, also a significant difference. In a prospective pilot evaluation of two fetuses, we measured the largest iliac angle at the most cephalad level of the iliac bones, and the smallest angle was measured at the most caudad level. The iliac angle measurements for the fetus with Down syndrome ranged from 55 degrees to 100 degrees; for the normal fetus, the range was 52 degrees to 80 degrees.

Although there is considerable overlap, the average iliac angle for fetuses with Down syndrome is significantly larger than that of normal fetuses. The angle measurement varies widely in any given fetus and is dependent on the level of the image. A prospective study is needed to evaluate the best level of angle measurement for the detection of fetuses with Down syndrome.