Measurement of femur length (FL) has been advocated as part of a genetic sonogram for the prediction of Down syndrome (DS). However its predictive ability has been inconsistent. We have studied the diagnostic value of this sonographic parameter in a prospective cohort of women with singleton gestations undergoing genetic sonogram between 14 and 22 weeks because of advanced maternal age or family history of aneuploidies. Genetic sonograms were performed at a mean gestational age of 17.0 weeks (range 14±22). DS was diagnosed in 30 fetuses, while 888 were euploid. MeantSD observed/expected (O/E) values of FL (1.00t0.10 versus 0.97t0.01, p=0.07) were not signi®cantly different between euploid and DS fetuses. Comparison of the regression equations of FL versus biparietal diameter revealed that while the intercepts were not signi®cantly different between euploid and DS fetuses, the difference in slopes reached signi®cance ( p=0.04) suggesting that the predictive ability of FL may increase with advancing gestational age. In addition, a MEDLINE search (National Library of Medicine) was conducted for articles published between 1985 and 1998 on fetal femur length in the prediction of trisomy 21. Review of the published literature on the subject suggests that FL is not a consistent or reliable sonographic predictor of DS.