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Nuchal translucency (NT) refers to an ultrasound measurement of the back of the fetal neck, which can be done from approximately 11 through 13 weeks of pregnancy. First-trimester screening combines the NT measurement with the results of a blood test, your age, and your ethnic background.
From these factors, a personal risk estimate is calculated. A positive result will identify 85% of Down syndrome cases and 80% of Trisomy 18 cases. In other words, if your fetus has Down syndrome, there is an 85% chance that the test will identify it. Likewise, if your fetus has Trisomy 18, there is an 80% chance the test will be positive for Trisomy 18. This test is a screen; in other words, it does not tell you definitively whether there is a fetal abnormality.
It simply determines the chances that your fetus has Down syndrome or Trisomy 18, and reveals whether those risks are high or low. The results are usually complete within 1 week. An abnormal first-trimester screen allows you to consider the option of the first-trimester diagnostic test, CVS (see later).