Validation of First Trimester Screening for Trisomy 21 in Singapore with Reference to Performance of Nasal Bone

Abstract

Introduction: The aim of this study is to describe the performance of first trimester screening (FTS) for trisomy 21 using maternal age, serum biochemistry and fetal nuchal translucency (NT) in a single center and to evaluate the effect of nasal bone on screening performance. Material and Methods: In 12,585 singleton pregnancies, the NT and nasal bone were examined. The majority of these mothers also had their serum biochemical markers analyzed. Risk was computed using different combinations of maternal age, biochemistry, NT and nasal bone. Down syndrome cases were confirmed by karyotyping. Results: There were 12,519 normal pregnancies, 31 with trisomy 21 and 35 with other chromosomal abnormalities. Without considering the nasal bone, the combined FTS detected 87.1% of trisomy 21 fetuses (false positive rate 5.1%), using 1:300 as the risk threshold, and this was further improved to 96.8% with the policy that classifies all fetuses with an absent nasal bone as high risk. Subgroup analysis showed that the detection rate would be 90.9%, with a false positive rate of 3.7%, if nasal bone was incorporated in the risk algorithm, compared to 81.8% and a false positive rate of 5.4% if it was not used. Discussion: FTS is very effective in early detection of trisomy 21 in Singapore. The nasal bone is a useful marker that can substantially improve the screening performance.