Nowadays, unlike the former generation, an expecting mother knows about most of the complications and issues that may emerge over the span of pregnancy. Therefore, she also realizes the importance of performing different prenatal screening tests.
These prenatal screening tests will help in noticing the majority of the issues with respect to a pregnancy, the health of the fetus and the wellness of the mother (especially if the pregnancy falls under the high-risk category) before it is too late to rectify. This will help the concerned people to make some significant decisions which will take the edge off some of the future complications (which otherwise arise unexpectedly leaving them more or less helpless).
Here, we explain one of such routine screening tests performed in the course of pregnancy – Nuchal translucency scan.
Nuchal Scan Test

What Is Nuchal Translucency Scan?

Nuchal fold is a spot at the base of the developing baby’s neck towards the back side. Nuchal translucency is an accumulation of fluid under the skin at the nuchal fold. Measuring the thickness of this fluid will help in assessing the risk of chromosomal abnormalities of the fetus.
The nuchal translucency scan, otherwise commonly called the NT scan, is a routine ultrasound performed to evaluate the risk of the developing fetus of having Down syndrome (DS) and some other chromosomal anomalies, and also major inborn heart issues.
NT scan reveals a clear space at the back of the developing baby’s neck indicating the translucent liquid which is collected at that part. Babies with irregularities have a tendency to accumulate more liquid at the back of their neck, bringing on this clear space to be bigger than normal. Thus, measuring the thickness of the fluid helps in assessing the risk of chromosomal abnormalities in the developing baby.

Why Is The Nuchal Scan Performed During Pregnancy?

NT scan is performed to determine the chances of a baby for Down’s syndrome or other chromosomal abnormalities which lead to congenital issues, particularly if the mother comes under risk group like being overage. It is recommended to perform this screening test before going for the diagnostic tests like amniocentesis (amniotic fluid tests) and CVS as these diagnostic tests have a potential risk of miscarriage. If the results of NT scan and blood tests (which is usually associated with NT scan) indicates no issues, then other unnecessary tests can be avoided.

When Is The Nuchal Scan Performed During Pregnancy?

The translucency of the fluid and transparency of the nuchal fold is a significant factor in determining the thickening of the fluid. Since the nuchal fold turns out to be less transparent as the infant develops, correct timing of NT scan is crucial. Usually, the NT scan is performed between the 11th and 14th weeks of pregnancy. Once you complete 14 weeks, then there is no use in performing this screening test. This is because by then the excess nuchal fluid will be absorbed by the lymphatic system of developing baby affecting the accuracy of the measurement.

Is The Nuchal Translucency Scan Result Accurate?

The accuracy of the NT scans depends on the expertise of the ultrasound technologist who performs it, the efficiency of the equipment and moreover the time during which the NT scan is performed.

NT scan is only a screening test rather than a diagnostic test. Thus, even if all the above conditions are satisfied, the result of an NT scan cannot be used to diagnose or confirm congenital issues in the fetus. Therefore never take a decision to terminate the pregnancy by just depending upon the NT scan result. The scan result only provides the level of risk that the baby possesses, which helps in deciding whether to have further diagnostic tests like chorionic villus sampling (CVS) or amniocentesis, which will confirm the chromosomal abnormalities or congenital issues that the baby may have. These tests will perform only if the NT scan result shows the increased probability of chromosomal abnormality of the baby.
Nuchal Test being performed
However, NT scan is found to detect 77 % of babies with Down’s syndrome.
The false positive rate is found to be around 5%. This means one in every twenty women the result will place the baby under the high-risk category even though the risk is actually low.

What Is The First Trimester Combined Screening?

Usually, the NT scan is performed along with some blood tests to get a more accurate result. The level of a pregnancy protein called PAPP-A in the blood of the mother is measured. If the level is lower during the first trimester of pregnancy the chances of genetic abnormalities and issues connected with the fetal heart are more. This method is called a ‘combined test’. Though the accuracy increases up to 90%, the false positive rate in this method is also 5%.

To Whom Is The Nuchal Translucency Scan Offered?

The NT scan is offered to all expecting mothers during their first trimester. It is now more often included as the part of the routine tests of the first trimester of pregnancy by the doctors. Though any women can have a baby with chromosomal defects, the risk increases with the maternal age.
For example, if the chances of a woman of carrying a baby with Down’s syndrome at the age of 25 is 1 in 1500, the chances of the same woman will increase to 1 in 150 once she is around 40 years old.
Thus, if an expecting mother is around 35 years of age (or more) NT scan is considered as unavoidable.

What Can I Know From The NT Scan Results?

The measurement of the baby should be between 45 – 84 mm and the pregnancy should be between the 11th and 14th weeks while the NT scan is performed.

  • The Nuchal translucency below 2.5 mm is generally considered as normal. The risk of Down’s syndrome and other chromosomal abnormalities increases with the measurement.
  • 9 out of 10 babies with the measurement 2.5 to 3.5 are found to be devoid of the chromosomal abnormalities.
  • A baby with a measurement of 6 mm possesses a higher risk of Down’s syndrome or congenital cardiac issues.
  • Sometimes the result will be in the form of a ratio expressing the chances of the baby to have a chromosomal defect. For example, 1 in 50 means for every 50 women with the same result one baby is detected with Down’s syndrome or other chromosomal defects. The same way, 1 in 6000 means, out of 6000 women with the same measurement as yours, one baby possessed Down’s syndrome. Therefore, higher the second number, lower will be the chances.

Whatever the screening results, you have to remember one thing. A normal measurement of nuchal fold does not guarantee a perfectly healthy child. The result just indicates that the child is “unlikely” to have health issues. Similarly, an abnormal measurement surely doesn’t indicate definite chromosomal issues. The results just show that the baby is more “likely” to possess chromosomal issues.