Humeral shortening in second-trimester fetuses with Down syndrome.

Individuals with Down syndrome have abnormally short long bones. Sonography has previously revealed slightly shortened femurs and thickened nuchal folds in affected second-trimester fetuses. Humeral and femoral lengths and nuchal folds were evaluated in 400 normal second-trimester controls and in 24 consecutive Down syndrome fetuses (14-20 weeks) undergoing amniocentesis for advanced maternal age or low maternal serum alpha-fetoprotein (MSAFP). Using the 400 normal control fetuses undergoing amniocentesis, the relationship between humeral length and biparietal diameter (BPD) was: Expected humeral length = -7.9404 + 0.8492 x BPD. A ratio of measured-to-expected humeral length of less than 0.90 identified 12 of 24 fetuses (50%) with Down syndrome and 25 of 400 normals (6.25%). Twelve of the 24 Down syndrome fetuses had a nuchal fold measuring 6 mm or larger. Combining nuchal fold findings with these humeral length criteria identified 18 of 24 affected fetuses (75%) without appreciable change in specificity, yielding a positive predictive value of 4.6% for women with a risk of one in 250 (age 35) of having a Down syndrome fetus. For women with risks of one in 500 and one in 1000 of having an affected fetus based on maternal age, a shortened humerus or thickened nuchal fold yielded positive predictive values of 2.3 and 1.2%, respectively.