How is Nuchal Translucency (NT) used to identify fetuses at increased risk for Down syndrome?

NT in combination with pregnancy associated plasma protein (P-APP) and the free beta subunit of human chorionic gonadotropin (§-hCG) constitutes the Ultra-screen test, which is used to detect fetuses with Down syndrome. The accuracy of the test is 90% and is the most effective test available for the first trimester. Performed between 11 and 14 weeks gestation, the Ultra-screen test involves an ultrasound assessment, which includes measurements of the NT as well as a blood test obtained from the mother. The results take about a week and when negative, reduce the risk for Down syndrome to an acceptable level. The ultra-screen test is available only for singleton and twin gestations. If a patient is carrying 3 or more fetuses she can still use NT by itself, which reduces the risk of Down syndrome by about 80%.