What is an NT scan and how does it help?

Pregnancy is a difficult phase replete with a lot of uncertainties, both for the mother and the fetus. But increased nuchal translucency thickness between 11-14 weeks of pregnancy has become synonymous with ‘morphological expression’ of fetal chromosomal anomalies and genetic syndromes. Further, it also serves as an indication of cardiovascular abnormalities. 

The nuchal scan or ​nuchal translucency ( NT) scan​ is a non-invasive sonographic prenatal scan ( ultrasound) that attempts to detect the risk of chromosomal abnormalities like Down’s Syndrome and other major visceral problems of the heart. 

The ​NT scan​ is carried out between 11-13+6 weeks of gestation since the baby’s neck ceases to be transparent after that due to shifting of fluid out of that area. This screening test checks the amount of fluid or the clear space of tissue at the back of the fetal neck. Babies affected with anomalies typically display the larger quantity of collected fluid in the first trimester. 

Like nuchal translucency which is measured at the end of the first trimester, the nuchal fold is measured during the second trimester and gives more concrete accuracy about the fetal viability. 

Development: 

In an NT scan, the fluid which is seen at the back of the neck might be the result of dropsy or fluid from lymphatic sacs due to abnormal embryological connections. 

The nuchal translucency is useful only till 14 weeks of pregnancy because the lymphatic system of the fetus is nascent and the resistance of placenta is high. However, after 14 weeks, the fetal lymphatic system becomes self-sufficient and drains away the excess fluid.

NT Scan as an Indicator: 

The NT scan is a boon to physicians and conceiving women as it estimates the odds of the fetus suffering from abnormal chromosomal, genetic or congenital problems. The common indications of anomalies found by NT scan are as follows: 

●     Turner Syndrome: Known as 45, X, Turner Syndrome is a chromosomal anomaly in females who have a missing X chromosome. 

●     Down Syndrome: Down Syndrome (Trisomy 21 ) is the most common chromosomal defect and the second most popular after Turner syndrome to be associated with nuchal translucency. The risk of babies suffering from Down syndrome increases by up to 14 times from maternal age 25-40. NT scan as a screening for Down syndrome was developed in the 1990s. 

Other common aftermaths of high nuchal translucency include:

●     Edwards Syndrome ( Trisomy 18 )

●     Patau Syndrome ( Trisomy 13 )

●     Triploid Syndrome

Process: 

The physician or sonographer first estimates the gestational age of the fetus by measuring it rump and stump. The NT scan is performed with the fetus in the sagittal plane. Then the sensor is placed on the mother’s abdomen to get the image on the monitor. Further, the nuchal translucency thickness is measured from edge to edge. 

The accuracy of the NT Scan: 

Like all screening tests, this particular one is not a definitive test and does not always give concrete results, but also helps with ​NT scan for stem cells​. The result of a scan may be far from reality. They might send out alarms when there is nothing to worry in a false positive and vice versa. However, the test is reliable as it detects 70-80% and 75% prenatal babies with Down Syndrome and Edwards Syndrome respectively, without erring.