Of the most common screening tests is known as the nuchal translucency test, or NT. Which is an ultrasound that measures the nuchal fold, which is the clear space in the tissue at the back of a developing babys neck. Babies with abnormalities tend to accumulate more fluid at the back of their neck during the first trimester, causing this clear space to be larger than average. The NT scan must be done when you're between 11 and 14 weeks pregnant. It's usually offered along with a blood test in what's known as first-trimester combined screening. Its important to understand that these screening tests only measure whether a baby is likely to have DS they do not offer a definitive diagnosis. For this reason, some parents decide to undergo more conclusive diagnostic tests. One diagnostic test is called chorionic villus sampling, or CVS, and involves extracting a small piece of a womans placenta for testing. Chorionic villus sampling is usually performed between the 10th and 12th weeks of pregnancy. Because CVS is an invasive test, there's a small risk about 1 in 100 that it can induce a miscarriage