A nuchal scan or nuchal translucency (NT) scan/procedure is a sonographic prenatal screening scan (ultrasound) to detect cardiovascular abnormalities in a fetus, though altered extracellular matrix composition and limited lymphatic drainage can also be detected. Since chromosomal abnormalities can result in impaired cardiovascular development, a nuchal translucency scan is used as a screening, rather than diagnostic, tool for conditions such as Down syndrome. However, increased nuchal translucency measurements are also associated with non-chromosomal abnormalities such as genetic conditions (e.g. Di George syndrome) and non-genetic ones (e.g. Body-stalk anomaly). The scan is carried out at 11–13+6 weeks pregnancy and assesses the quantity of fluid collecting within the nape of the fetal neck. There are two distinct measurements – the nuchal translucency, which is measured earlier in pregnancy at the end of the first trimester, and for which there is a lower threshold for increased diameter, and the nuchal fold, which is measured towards the end of the second trimester. The scan may also help confirm both the accuracy of the pregnancy dates and the fetal viability. As nuchal translucency size increases, the chances of a chromosomal abnormality and mortality increase; 65% of the largest translucencies (>6.5mm) are due to chromosomal abnormality, while fatality is 19% at this size.
NB stands for nasal bone. So during the NT scan, they are looking at the thickness of the nuchal fold and whether the baby has a nasal bone yet. An overly thick nuchal fold and/or absence of the nasal bone can be markers for Down syndrome. The NT scan is usually performed in conjunction with a blood test taken around the same time. You will get a screening result giving you the odds of your having a baby with a chromosomal disorder. For example, a 1 in 2500 chance of having a baby with T21 (Down syndrome). Note that the NT scan / bloods are screening only. It can only tell you whether you have an increased chance of a chromosomal disorder. To find out a definite yes or no, you would have to do a diagnostic test (eg: MaterniT21, CVS or amnio).