A nuchal translucency scan, also known as an NT scan, is a prenatal screening test done by ultrasound that helps to calculate the baby’s risk of being born with Down syndrome and certain other chromosomal disorders. The NT scan measures the amount of fluid behind the fetus’ neck. About 65 to 85 percent of fetuses with chromosomal disorders will have excess fluid. The scan is most accurate when it’s carried out between weeks 11 and 13. It’s important to know, though, that an NT scan can only tell you the relative risk of a baby having a chromosomal disorder. The test is not diagnostic. The only way to know for sure whether or not the fetus has a genetic abnormality is by having a diagnostic test such as an amniocentesis or chorionic villus sampling. Newer tests are also beginning to be available that can diagnose genetic abnormalities using only a maternal blood sample.