Can fetal iliac bone measurement be used as a marker for Down's syndrome screening?

Abstract

The aim of this study was to assess the application of iliac bone length measurement in screening for Down's syndrome. Measurement of fetal iliac bone length was prospectively carried out in 609 pregnant women at the time of amniocentesis, and correlated with the results of karyotype analysis. Following exclusion of cases with chromosomal abnormalities other than trisomy 21, cases with intrauterine growth impairment or structural abnormalities and cases with incomplete follow-up, 466 karyotypically normal fetuses and 14 fetuses with trisomy 21 were included in the analysis. Centiles, standard deviations, expected values by means of linear regression analysis of the iliac bone, in relation to the biparietal diameter, were calculated in the normal fetuses. In order to identify pregnancies at risk for trisomy 21, where a 'longer' than normal iliac bone was expected, three cut-offs previously proposed by other authors and three new criteria were employed. Sensitivity, false-positive rate and likelihood ratio were calculated to assay the different cut-off criteria. Eleven of the 14 (79%) fetuses affected by trisomy 21 had an iliac bone length greater than the 50th centile of normal values. The most useful threshold was an iliac bone length > or = 2 SD of the normal (29% sensitivity and 2% false-positive rate). This study confirms that fetuses with Down's syndrome tend to have a longer iliac bone length measurement than those with normal karyotype. However, due to the different methods used in different centers, the application of this measurement does not offer sufficiently convincing results for it to be used when screening for Down's syndrome.