Ultrasonically evident fetal nuchal skin thickening: is it specific for Down syndrome?

Fetal nuchal thickening in the second trimester is suggested as an ultrasonographic sign strongly suggesting the presence of Down syndrome. To better understand this potentially valuable observation, we explored aspects of the finding not considered in previous reports. The published data were reanalyzed to estimate statistically the presence of the sign in a low-risk obstetric population. Second, films of chromosomally normal fetuses were reviewed to confirm the normal range of measurement. Finally, autopsy reports of aborted Down syndrome fetuses were reviewed. Our results confirm that true nuchal thickening is present in some second-trimester Down syndrome fetuses. Unfortunately, a positive sign occurs in a significant proportion of chromosomally normal fetuses. Also our calculations predict a high incidence of false positive results in a low-risk obstetric population. Consequently we urge caution in the interpretation of these findings until prospective characterization of these observations is available.