Prospective multicenter study of second-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 this sonographic measurement as a screening test for fetal Down syndrome.

Methods: 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.

Results: 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%.

Conclusion: In this investigation, excess nuchal skinfold thickness was a poor and unreliable screening test for Down syndrome.