Screening for trisomy 21: the significance of a positive second trimester serum screen in women screen negative after a nuchal translucency scan

Abstract

Screening of pregnancies for trisomy 21 is now an accepted part of antenatal care. Measurement of fetal nuchal translucency in the first trimester and analysis of maternal serum biochemistry in the second trimester are both established methods of screening. The performance characteristics of both tests in an unselected population are well described and the choice of test offered is usually determined by local policy and resources. We present data from a screening programme offering women with a low risk result from nuchal translucency measurement a second trimester serum screen. There were eight cases of trisomy 21 in the 2683 women screened, all of which presented with a high-risk nuchal screen result. Serum screening of 1057 women who screened negative by nuchal translucency gave 46 high-risk results, all of which were, therefore, false positive for trisomy 21. Second trimester biochemistry screening following a negative nuchal translucency screen did not increase the detection of trisomy 21.