Nuchal translucency

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 cardiovasular 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.

All women, whatever their age, have a small risk of delivering a baby with a physical or cognitive disability. The nuchal scan helps physicians estimate the risk of the fetus having Down syndrome or other abnormalities more accurately than by maternal age alone.

Overall, the most common chromosomal disorder is Down syndrome (trisomy 21). The risk rises with maternal age from 1 in 1400 pregnancies below age 25, to 1 in 350 at age 35, to 1 in 100 at age 40. Down syndrome is the second most common chromosomal abnormality associated with increased nuchal translucency, after Turner syndrome (45,X).

Until recently, the only reliable ways to determine if the fetus has a chromosomal abnormality was to have an invasive test such as amniocentesis or chorionic villus sampling, but such tests carry a risk of causing a miscarriage estimated variously as ranging between 1% or 0.06%. Based on maternal age, some countries offer invasive testing to women over 35; others to the oldest 5% of pregnant women. Most women, especially those with a low risk of having a child with Down syndrome, may wish to avoid the risk to the fetus and the discomfort of invasive testing. In 2011, Sequenom announced the launch of MaterniT21, a non-invasive blood test with a high level of accuracy in detecting Down syndrome (and a handful of other chromosomal abnormalities). As of 2015, there are five commercial versions of this screen (called cell-free fetal DNA screening) available in the United States.

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