Prospective multicenter study of second-trimester nuchal skinfold thickness in unaffected and down syndrome pregnancies

To determine the distribution of nuchal skinfold thickness in normal and Down syndrome pregnancies and to evaluate the use of this sonographic measurement as a screening test for fetal Down syndrome. A prospective, multicenter, population-based study was performed by experienced obstetric sonographers on 1382 women with sonographically normal fetuses undergoing second-trimester amniocentesis for the indication of advanced maternal age. A standard, well-defined sonographic image was obtained at all collaborating centers. The distributions of nuchal skinfold thickness were compared between euploid and Down syndrome fetuses. There were 1346 chromosomally normal pregnancies, 13 fetuses with Down syndrome (1:106), and 23 other chromosome abnormalities. Seventeen fetuses had measurements of 6 mm or greater, and one of these had Down syndrome. The median nuchal skinfold thickness 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 significant difference in nuchal skinfold between the euploid and Down syndrome fetuses ( P =.5). Overall, using a nuchal skinfold thickness of 6 mm or greater as a screening test, the detection rate for Down syndrome was one of 13 (8%), the false-positive rate was 16 of 1382 (1.2%), the positive predictive value was one of 17 (6%), and the probability of detecting Down syndrome was 6.5%. In this investigation, excess nuchal skinfold thickness was a poor and unreliable screening test for Down syndrome.