ADJUSTING THE RISK FOR TRISOMY-21 BY A SIMPLE ULTRASOUND METHOD USINGFETAL LONG-BONE BIOMETRY

Objective: To establish the efficacy of second-trimester fetal long-bo ne biometry (femur, humerus, tibia, and fibula length) in detecting tr isomy 21 and to generate tables for adjusting the risk of trisomy 21 a ccording to long-bone biometry. Methods: Four long bones-femur, humeru s, tibia, and fibula-were measured ultrasonically in singleton fetuses before genetic amniocentesis. Fetuses with normal karyotypes were use d to derive regression equations describing predicted lengths on the b asis of the biparietal diameter measurement. The efficacy of each abno rmally short bone, alone and in combination, was determined in 22 fetu ses with trisomy 21 encountered during the study period. After the sen sitivity and specificity of long-bone biometry were established, appro priate tables were generated by Bayes' theorem to adjust the risk of t risomy 21 in the second trimester depending on long-bone biometry. Res ults: Of 515 patients between 14 and 23 weeks' gestation, 493 had norm al fetal karyotypes and 22 had trisomy 21. The sensitivity of an abnor mal ultrasound, as defined by the presence of one or more short bones, was 63.6% and the specificity was 78.5%. According to Bayes' theorem, genetic amniocentesis may not be recommended for women less than 40 y ears old in the presence of normal long-bone biometry (ie, all four bo nes normal). Conclusion: Second-trimester fetal long-bone biometry is useful in detecting trisomy 21 and may be used to adjust the a priori risk of both high- and low-risk women for trisomy 21 and, therefore, t he need for genetic amniocentesis.