Combining humerus and femur length for improved ultrasonographic identification of pregnancies at increased risk for trisomy 21

OBJECTIVE: Our purpose was to evaluate the value of the combination of femur and humerus length measurements in ultrasonographic screening for trisomy 21. STUDY DESIGN: Direct necropsy measurements were analyzed on 703 midgestational fetuses (641 normal, 62 with trisomy 21). The (leg + arm length)/foot length ratio was found to be significantly shortened for fetuses with trisomy 21. On the basis of necropsy data 576 midgestational pregnancies were evaluated ultrasonographically for (fermur - humerus length)/foot length ratio to identify fetuses at increased risk for trisomy 21. RESULTS: An ultrasonographic (femur + humerus length)/foot length ratio ≤ 1.75 gave a 15.3 odds ratio risk for trisomy 21 in our high-risk population and correctly identified 53% of fetuses with trisomy 21, with a false-positive rate of 7%. In addition, the use of this ratio eliminates the need for gestational age-corrected nomograms and complicated calculations in ultrasonographic screening. CONCLUSION: The (femur + humerus length)/foot length ratio may be an additional effective ultrasonographic marker for identification of fetuses at increased risk for trisomy 21.