Agreement between predicted risk and prevalence of Down syndrome in first trimester nuchal translucency screening.

The agreement between predicted risks of Down syndrome and observed prevalence was investigated in a population of 11,847 singleton pregnancies screened by first trimester nuchal translucency at a single institution. Twenty-seven cases of Down syndrome were observed; 20 were detected prenatally by nuchal translucency and maternal age screening, three by other means and four postnatally. The screened women were grouped according to their predicted risk of having an affected pregnancy, and this was compared with the observed prevalence. A significant correlation between predicted and observed prevalences was noted, thus demonstrating that risk estimates for Down syndrome based on first trimester nuchal translucency screening are accurate.