The second-trimester fetal iliac angle as a sign of Down's syndrome

To determine whether the iliac angle of second-trimester fetuses is a useful sonographic marker for the detection of fetuses with Down's syndrome.

We prospectively measured the iliac angle for all fetuses karyotyped over a 17-month period. A transverse view of the upper fetal pelvis was obtained at the time of amniocentesis and, by means of a protractor, the angle between the two iliac bones was measured. The angles of all those fetuses with Down's syndrome and the first 500 fetuses with normal karyotypes were compared, as was gestational age and maternal age.

Nineteen fetuses with Down's syndrome and 1167 fetuses with normal karyotypes underwent genetic amniocentesis over the study period. The mean gestational age of the fetuses with Down's syndrome was 16.6 weeks (+/- 1.9 weeks), and for the controls, 16.1 weeks (+/- 1.4 weeks) (p = 0.3). The mean maternal ages were 34.7 years (+/- 4.8 years) and 35.4 years (+/- 4.1 years) for those pregnancies with Down's syndrome and with normal karyotypes, respectively (p = 0.5). The mean iliac angle was 80.1 degrees (+/- 19.7 degrees) for those fetuses with Down's syndrome, and 63.1 degrees (+/- 20.3 degrees) for those fetuses with normal karyotypes (p = 0.0004). The iliac angle was at least 90 degrees in 36.8% (seven of 19 fetuses) with Down's syndrome and in 12.8% (64 of 500 fetuses) with normal karyotypes (p = 0.003).

Fetuses with Down's syndrome have a significantly larger iliac angle than fetuses with normal karyotypes. Use of an iliac angle cut-off point of 90 degrees would identify over one-third of second-trimester fetuses with Down's syndrome. However, because of the high false-positive rate (12.8%), the iliac angle is not useful in a high-risk population as a screening test when used in isolation.