Femur/foot length ratio for detection of Down syndrome: results of a multicenter prospective study. The Association Française pour le Dépistage et la Prévention des Handicaps de l'Enfant Study Group.

Abstract

The aim was to determine the sensitivity and specificity of a short femur for detection of trisomy 21 in the second trimester. Thirty-six investigators in 12 centers measured biparietal diameter and femur and foot lengths in 3582 14- to 24-week-old fetuses in mothers undergoing amniocentesis for age, history of genetic disorder, or laboratory signs. Among the various ratios for evaluating femur shortening the femur/foot ratio appeared to be the most discriminatory. At an upper cutoff level of 0.88 a sensitivity of 35% was obtained for 4.6% false positives in normal infants. However, to only obtain 2.3% false positives, the cutoff limit had to be set at 0.85, giving a sensitivity of no more than 15%. Determination of the femur/foot ratio improves ultrasonographic detection of trisomy 21 in the second trimester, although for systematic use it would lead to an unacceptable number of unnecessary amniocenteses.