FETAL LEG AND FEMUR FOOT LENGTH RATIO - A MARKER FOR TRISOMY-21

OBJECTIVE: Our purpose was to determine whether the femur/foot length ratio is useful in the prenatal detection of trisomy 21. STUDY DESIGN: Direct necropsy measurements were analyzed on 436 midgestational fetuses (391 normal, 45 with trisomy 21). Necropsy leg/foot length ratio versus gestational age was found to be significantly different between normal fetuses and those with trisomy 21. On the basis of the necropsy data, 345 midgestational pregnancies were evaluated ultrasonographically for femur/foot length ratio versus gestation age to identify fetuses at risk for trisomy 21. RESULTS: A leg/foot length ratio versus gestation age less-than-or-equal-to 2.3 correctly identified 84% of fetuses with trisomy 21 between 110 to 155 days gestation age at necropsy. An ultrasonographic femur/foot length ratio versus gestational age less-than-or-equal-to 0.9 had an 18.3 odds ratio risk for trisomy 21 in our high-risk population and correctly identified 71% of fetuses with trisomy 21 (positive predictive value 0.24, negative predictive value 0.98). CONCLUSION: The femur/foot length ratio is an additional ultrasonographic marker for identification of fetuses at increased risk for trisomy 21.