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 f or trisomy 21. STUDY DESIGN: Direct necropsy measurements were analyze d on 703 midgestational fetuses (641 normal, 62 with trisomy 21). The (leg + arm length)/foot length ratio was found to be significantly sho rtened for fetuses with trisomy 21. On the basis of necropsy data 576 midgestational pregnancies were evaluated ultrasonographically for (fe mur + humerus length)/foot length ratio to identify fetuses at increas ed risk for trisomy 21. RESULTS: An ultrasonographic (femur + humerus length)/foot length ratio less than or equal to 1.75 gave a 15.3 odds ratio risk for trisomy 21 in our high-risk population and correctly id entified 53% of fetuses with trisomy 21, with a false-positive rate of 7%. In addition, the use of this ratio eliminates the need for gestat ional age-corrected nomograms and complicated calculations in ultrason ographic screening. CONCLUSION: The (femur + humerus length)/foot leng th ratio may be an additional effective ultrasonographic marker for id entification of fetuses at increased risk for trisomy 21.