Nuchal Translucency Screening Test: How Accurate is this Test?

Nuchal Translucency Scan
Editor's Note: This article has been recently updated with latest information and research studies.
 

Pregnancy is the state where a woman has conceived in her womb a child. It is a crucial stage and part of a women’s life. Many parents, especially new parents, are anxious to know whether their child is healthy or, more importantly, normal. There are prenatal tests that have been developed to be able to identify the baby’s gender and condition. But what makes parents more curious is that whether their child is normal or not. It will always cross the parents’ minds that their child might have a medical condition when he/she comes out. One common condition that a child might have is Down syndrome or Trisomy 21. To know, not to diagnose, whether there is a possibility that the child has Down syndrome, a prenatal screening test is a widely accepted tool to detect any fetal conditions during pregnancy. This test is called the Nuchal Translucency test that is conducted during the first trimester of the mother’s pregnancy.

What is the Nuchal Translucency Screening Test?

Nuchal translucency screening test is a prenatal ultrasound screening test done to pregnant women who are in their first trimester, around 1014 weeks of pregnancy. It is characterized by the thickness of the translucent space between the skin and the soft tissue overlying the cervical spine that is measured in millimeters by an ultrasound. The measurement of the nuchal translucency has been found to be a useful indication of Down syndrome in the late first trimester. The increase* in the thickness of the nuchal translucency is an indication of fetal chromosomal abnormalities. Because of the sophistication of this tool, experts are needed to operate the machines used for this test. According to the American Pregnancy Association, the chances of having a baby with a Down syndrome or Trisomy 21 defect increases* with age because of the higher risk of the chromosomes to divide incorrectly. The chances of having a Down syndrome baby is usually at the maternal age of 35.

Nuchal Translucency Screening

Although it has been already used extensively, its affectivity is still being studied and developed based on the operator of the NT machine. A study conducted by the Fetal Medicine Unit of the Department of Obstetrics and Gynecology in Institute Universities Dexeus in Spain about the Reliability of the nuchal translucency measurements. The study involves pregnant women who were in their first trimester. The machines were operated by different operators that consist of seven out of 20 operators (35%) had expertise with fetal medicine, and 2 of them (10%) were FMF certified. Results show that even if the operator has the proper practice, the results would still vary but of reliable level.

Because this prenatal ultrasound screening test does not tell whether the child has down syndrome or not, it is often supplemented with the Noninvasive Prenatal Testing (NIPT). NIPT is a simple blood test that would analyze the cell-free DNA or cfDNA to know the possibility of the child of having genetic disorders like Down syndrome. Even though it does not accurately tell about the child’s abnormalities, it is considered to be highly accurate.

Advantages of Nuchal Translucency Screening Test

The NT test gives you the advantages of being able to examine whether your child is at risk for chromosomal problems at the earlier stages of pregnancy not having to subject yourself to do invasive test like chorionic villus sampling (CVS). But if results from the NT screening shows a high risk, the mother might be prescribed to do CVS. Since the NT test is considered as a safer and non-invasive test for chromosomal abnormalities, making this the first option lowers the number of a procedure associated fetal loss. Also, knowing your child’s condition in the first trimester of your pregnancy, would help the parents plan as to how they will take care of the affected child.

How is the Nuchal Translucency Test Screening Done?

The nuchal translucency test is performed using a trans-abdominal ultrasound that involves scanning through your lower abdomen. A small amount of ultrasound gel is placed on the lower abdomen of the mother. The gel helps improve* contact between the probe and the skin. The ultrasound probe is then used to scan through the gel.

Sometimes a trans-vaginal ultrasound is needed. It is an internal ultrasound scanning test where the ultrasound probe is lying in the vagina. The probe is thin with a diameter of 2cm and it is covered with a disposable protective sheath. The same procedure is done like that in trans-abdominal ultrasound. A small amount of gel is placed on the end of the probe. The probe is then gently inserted at a short distance into the vagina. This procedure usually causes less discomfort for the patient.

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How are the Results Computed?

The results are computed based on some factors namely the mother’s age, nuchal translucency measurement of the baby, gestational age of the baby and blood tests. There are two risk factors that are computed for this test.

  • Background Risk: The background risk is the starting point for assessing the certain risk for the baby. This risk is calculated mainly based on the mother’s age and gestational age of the child. If you have had a baby with the chromosomal abnormality, then the background risk is increased. The result of the background risk is usually presented as “1 in a …” risk. For example, the result would indicate that your background risk for Down syndrome may be “1 in 500”.
  • Adjusted Risk: The adjusted risk is the risk for the child that is calculated using the nuchal translucency measurement and the special blood tests.

Must Watch – Nuchal Translucency Scan

What do the Results Mean?

The results are usually presented as ratios such as “1 in 500”. This ratio indicates that there is a 1 in a 500 chance of having a baby with Down syndrome and a 499 in 500 chance of having a baby without.

If the result of your adjusted risk is below 1 in 300, such as 1 in 600 or 1 in 1000, then it is a “low risk”. A low risk, however, does not necessarily mean that there is no chance for the mother to have a baby with Down syndrome. This is an indication that the child is healthy. If the results are higher than 1 in 300, such as 1 in 250 or 1 in 50, then it is more likely that you will have a child with Down syndrome and it is called “high risk”. However, this is still not an indication that your child has a chromosomal disorder. Further tests may be prescribed to confirm such as CVS.

How Accurate is this Test?

The accuracy of the NT test alone is estimated at 70 – 75% accurate. For better results, it is usually recommended to combine the NT ultrasound test with one or two blood tests to would measure and compare the results with two maternal pregnancy hormones, beta hCG and PAPP. The combination of these tests will increase* accuracy to 83 – 92% accuracy.

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Author

Expert Author : Fouzia Khan (Consumer Health Digest)

Fouzia Khan is a graduate from University of Lethbridge. She has years of teaching and research experience in Computer Science and Engineering. Apart from teaching she is passionate about conducting research, writing, and sharing new innovations of science with others.