The current role of sonography in the detection of Down syndrome.

The biparietal diameter (BPD)/femur length ratio, nuchal skin thickening, and measured-to-expected femur length ratio have been suggested as sonographic predictors of Down syndrome. In an effort to validate these parameters, we compared their individual and combined prevalence in 22 fetuses with Down syndrome. Using our normative data, the sensitivity and specificity of the BPD/femur length ratio were 36.4 and 93.4%, respectively. Assuming an incidence of Down syndrome in the general population of one in 1000, the estimated positive predictive value was 0.6%. When the BPD/femur length ratio, nuchal skin thickening, and measured-to-expected femur length ratio were combined, the sensitivity was improved (36.4 versus 45.5%) without significantly altering the specificity (93.4% versus 92.3%). A large prospective study is required to verify the utility of sonographic predictors of Down syndrome before their application can be recommended.