First-trimester screening for Down syndrome using nuchal translucency measurement with free β-hCG and PAPP-A between 10 and 13 weeks of pregnancy-the combined test

In a population of 1467 women attending the ‘G. Gaslini’ Institute for antenatal care, we evaluated first‐trimester risk screening for Down syndrome using the ‘combined test’ based on ultrasound measurement of nuchal translucency (NT), maternal serum pregnancy‐associated plasma protein A (PAPP‐A) and free β‐hCG, and maternal age. No clinical action was taken on these results. The gestational age, determined by scan measurement of crown–rump length, ranged from 10 weeks to 13 weeks 6 days. The median maternal age was 31 years 8 months. There were 13 Down syndrome pregnancies. The risk of having an affected pregnancy was estimated from a multivariate Gaussian distribution, using commercially available software. With a risk cut‐off of 1 in 350, 11 affected pregnancies were detected (detection rate 85 per cent, 95 per cent confidence interval: 56–100 per cent) with a 3.3 per cent false‐positive rate. The odds of being affected given a positive result were 1 in 30. Further data are needed to determine, with greater statistical reliability, the relative performance of the combined test with current second‐trimester screening. Copyright © 1999 John Wiley & Sons, Ltd.