A nuchal scan or nuchal translucency (NT) scan is a sonographic prenatal screening scan (ultrasound) to detect cardiovascular abnormalities in a fetus. Chromosomal abnormalities can lead to impaired cardiovascular development. Screening of these abnormalities is therefore very important. NT scan is used as a screening tool rather than a diagnostic tool for conditions like Down’s Syndrome. The scan is carried out at 11–13 weeks into pregnancy and assesses the quantity of fluid collecting within the nape of fetal neck. There are two distinct measurements. The first one being nuchal translucency, which is measured earlier in pregnancy at the end of the first trimester for which there is a lower threshold for increased diameter. The second one, 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.
Nuchal scan is performed between 11 and 14 weeks of gestation, because the accuracy is best in this period. The scan is obtained with the fetus in sagittal section and a neutral position of the fetal head (neither hyper flexed nor extended, either of which can influence the nuchal translucency thickness). The fetal image is enlarged to fill 75% of the screen, and the maximum thickness is measured, from edge to edge. It is important to distinguish the nuchal lucency from the underlying amniotic membrane. Normal thickness depends on the crown-rump length (CRL) of the fetus. Among those fetuses whose nuchal translucency exceeds the normal values, there is a relatively high risk of significant abnormality.
The actual anatomic structure whose fluid is seen as translucency, is likely the normal skin at the back of the neck, which either may become edematous or in some cases, filled with fluid by dilated lymphatic sacs due to altered normal embryological connections.
The translucent area measured (the nuchal translucency) is only useful between 11 and 14 weeks of gestation, when the fetal lymphatic system is developing. After 14 weeks, the lymphatic system is likely to have developed sufficiently to drain away any excess fluid. So after this time, any abnormalities causing fluid accumulation may seem to correct themselves and can thus go undetected by nuchal scanning. The buildup in fluid is due to a blockage of fluid in the developing fetal lymphatic system. Progressive increase in the width of the translucent area during the 11 to 14 week measurement period is thus indicative of congenital lymphedema.
Why is it important to get a NT scan?
NT scan is useful in detecting chromosomal abnormalities. Chromosomal abnormalities lead to a lot of genetic disorders like Down’s syndrome, Turner’s syndrome, etc. which leads to severe health issues. Therefore, it is important to go through the procedure. Just to be prepared if there is any abnormalities in the chromosomal number of the fetus. NT scan also helps in detecting impaired cardiovascular developments which is also caused by chromosomal abnormalities.
The results may not be accurate in all the cases but the precision is quite high. It is better to stay on the safe side. Early screening of abnormalities will help in the diagnosis and treatment, if any.