Nuchal translucency measurement, or NT scan, is a prenatal screening method that allows your doctor to determine the risk of Down syndrome (DS) and other chromosomal abnormalities in an unborn baby.
Tiny translucent areas on the back of the fetal neck are measured by ultrasound. These areas can be seen even before your pregnancy is confirmed. The thickness of these regions, called nuchal translucency (NT), is measured according to established standards. The thicker the NT measurements, the higher the risk of DS or other chromosomal disorders in your baby.
This first trimester screening is performed between 11 weeks and 13 weeks and 5 days. It is non-invasive and painless, unlike amniocentesis which carries a small risk of miscarriage due to the needles used.
The NT screening is usually combined with blood tests (MSAFP or Quad screen). These tests analyze maternal blood for evidence of chromosome abnormalities. The blood test results may not only complement the results from the NT scan but also give more accurate results if there is any abnormality present in fetus. The combination of the two can be used to establish your baby’s risk factor for Down syndrome, Trisomy 18 and open neural tube defects like spina bifida. When taken together, these tests reduce your chances of miscarriage from false positive results to 1%.
There may be some discomfort involved in this test, although many women will feel nothing at all. A small amount of gel will be spread across your abdomen and then quickly wiped off again before the transducer is placed against your skin. Your baby’s heartbeat will then be monitored as the ultrasound probe moves over your stomach.
Your baby’s neck will be checked for any signs of fluid (the nuchal translucency) that might indicate a chromosomal abnormality such as Down’s syndrome. The measurements taken from this fluid should correspond with those taken from other tests, such as blood tests and measurements of fetal movements, to provide an accurate assessment.
NT screening has been found to be between 65-85% accurate in detecting Down syndrome and 95% accurate in detecting trisomy 18. Occasionally, it will detect a false positive; that means that everyone appears normal in all their chromosomes except one, which causes concern. However, this only occurs less than 1% of the time. Because of these percentages being so high, most doctors will recommend having a CVS or amniocentesis once you reach your second trimester if the results from the NT screening show any abnormalities.