Why is ultrasound screening offered in the first trimester?
First trimester nuchal translucency (NT) ultrasound is a non-invasive screening method used to identify babies with an increased risk for chromosomal abnormalities such as Down syndrome. This assessment is done between 11+0 and 14 weeks gestation by measuring the thickness of a skin fold at the back of the neck called the NT. The baby’s length, NT and your age are combined to calculate your baby’s risk level of Down syndrome (Trisomy 21). A blood test measuring two hormones (PAPPA and HCG) can also be taken. If used together with the ultrasound measurements it can improve accuracy of the test. The detection rate for Down syndrome using NT and your age is 75-80%. By adding the blood test results, the detection rate is 85-90%. However it is important to remember that not all babies with a chromosomal abnormality will be detected by this method as the detection rate is not 100% guaranteed using either technique.
