Femur/foot length ratio for detection of down syndrome: Results of a multicenter prospective study

OBJECTIVE: The aim was to determine the sensitivity and specificity of a short femur for detection of trisomy 21 in the second trimester. STUDY DESIGN: 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. RESULTS: 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 obstained 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%. CONCLUSION: Determination of the femur/foot ratio improves ultrasonographic detection of trisomy 21 in the second trimester, although for systemic use it would lead to an unacceptable number of unnecessary amniocenteses.