PROSPECTIVE MULTICENTER STUDY OF 2ND-TRIMESTER NUCHAL SKINFOLD THICKNESS IN UNAFFECTED AND DOWN-SYNDROME PREGNANCIES

Objective: To determine the distribution of nuchal skinfold thickness in normal and Down syndrome pregnancies and to evaluate the use of thi s sonographic measurement as a screening test for fetal Down syndrome. Methods: A prospective, multicenter, population-based study was perfo rmed by experienced obstetric sonographers on 1382 women with sonograp hically normal fetuses undergoing second-trimester amniocentesis for t he indication of advanced maternal age. A standard, well-defined sonog raphic image was obtained at all collaborating centers. The distributi ons of nuchal skinfold thickness were compared between euploid and Dow n syndrome fetuses. Results: There were 1346 chromosomally normal preg nancies, 13 fetuses with Down syndrome (1:106), and 23 other chromosom e abnormalities. Seventeen fetuses had measurements of 6 mm or greater , and one of these had Down syndrome. The median nuchal skinfold thick ness in Down syndrome was 3.2 mm and in euploid fetuses was 3.1 mm. By the Mann-Whitney rank-sum test, there was no statistically significan t difference in nuchal skinfold between the euploid and Down syndrome fetuses (P = .5). Overall, using a nuchal skinfold thickness of 6 mm o r greater as a screening test, the detection rate for Down syndrome wa s one of 13 (8%), the false-positive rate was 16 of 1382 (1.2%), the p ositive predictive value was one of 17 (6%), and the probability of de tecting Down syndrome was 6.5%. Conclusion: In this investigation, exc ess nuchal skinfold thickness was a poor and unreliable screening test for Down syndrome.