Down's syndrome: First trimester approach

Objectives: The aim of our study was to evaluate the "combined test", a new first trimester screening test for Down's syndrome based on maternal age, ultrasound measurement of nuchal translucency thickness (NT), and maternal serum levels of pregnancy-associated plasma protein A (PAPP-A) and free β-hCG. Methods: The population attending our prenatal diagnosis unit was offered a first trimester scan. Nuchal translucency thickness measurements were performed by transabdominal ultrasound at 10-13 weeks of gestation. A blood sample was taken, and immunoradiometric assays for PAPP-A and free β-hCG performed. The risk of having an affected pregnancy was estimated from a multivariate Gaussian distribution, using a commercially available software. Results: A total of 1836 singleton pregnancies were enrolled. The gestational age, determined by ultrasound 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 16 cases of Down's syndrome pregnancies. With a risk cut-off of 1 in 350, 14 affected pregnancies were detected (detection rate 87%, 95% confidence interval: 62-98%) with a 3.3 percent false-positive rate. The odds of being affected given a positive result were 1 in 30. Conclusions: Our results confirmed the value of the combined test in first-trimester screening for Down's syndrome.