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This prenatal test (also called the NT or nuchal fold scan) can help your healthcare practitioner assess your baby’s risk of having Down syndrome (DS) and some other chromosomal abnormalities, as well as congenital abnormalities.

The NT test uses ultrasound to measure the clear (translucent) space in the tissue at the back of your developing baby’s 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 13 weeks pregnant. (The last day you can have it done is the day you turn 13 weeks and 6 days pregnant.) It’s usually offered along with a blood test to estimate the risk of your baby having down syndrome, trisomy 18, or spina bifdia (open spine) in what’s known as first-trimester combined screening.

Like other screening tests, an NT scan won’t give you a diagnosis. But it can assess your baby’s risk for certain problems and help you decide whether you want to have chorionic villus sampling (CVS) or amniocentesis to find out whether your baby is actually affected.