Nuchal Translucency (NT) Scan during Pregnancy

Congenital disorders or defects are defined as disorders or defects that babies are born with, or in simpler words – birth disorders or defects. Most of the time, congenital disorders cannot be treated. However, it definitely helps to know before your baby is born, if your baby is going to have any such disorder. Diagnosis tests have hence been designed for several congenital defects. One such test is the Nuchal Translucency (NT) Scan.

What is NT Scan?

The Nuchal Translucency (NT) Scan is a prenatal screening test. It allows pregnant women to determine primarily the risk factor of their baby being born with Down’s Syndrome. Down’s Syndrome is a congenital defect wherein a baby is born with three copies of chromosome number 21 (instead of the normal 2 copies).

Along with this, the NT scan is also able to detect if your baby is at the risk of having few other congenital defects like other chromosomal abnormalities (particularly in chromosome number 13 and 18), and heart problems.

Remember that the NT scan is only a ‘screening’ test. It cannot identify with 100% accuracy whether or not your baby will be born with Down’s Syndrome. It will only identify the degree of ‘risk’. In other words, the NT scan gives a probability or ‘chance’ of your baby developing Down’s Syndrome. Based on the results of the NT scan (which will be explained further in this article) your gynaec may/may not advise you to take a ‘diagnostic’ test. A diagnostic test will tell you with certainty whether or not your baby will develop Down’s Syndrome.

How is Nuchal Translucency Scan Done?

The NT scan is basically an ultrasound test.

What is the Nuchal Translucency?

The nuchal translucency is nothing but the degree of translucency of the fluid that builds up at the back of your baby’s neck (or the ‘nape’ of the neck), during its normal growth and development within your womb. The fluid build up is a normal process. However, the composition of this fluid – especially its thickness and translucency – is affected by chromosomal abnormalities, such as the one responsible for Down’s Syndrome.

How is Nuchal Translucency Ultrasound scan done?

An NT scan is nothing but a sonography. It can be performed as a Transabdominal or Transvaginal Ultrasound.

1. Transabdominal Ultrasound:

• a scoop of the ultrasound gel will be applied to your lower abdomen
• the ultrasound probe is placed on the gel to ‘scan’ your baby
• if the results of this ultrasound are not satisfactory enough for your sonographer to build a report, you may be requested to go for a transvaginal ultrasound
• you need a partially full abdomen to perform this test; consult your sonographer about the amount of fluids you should consume before this test

2. Transvaginal Ultrasound: while this may sound more ‘invasive’, it is an assuredly safe procedure.

• a thin ultrasound probe is used for this test; the probe is about 2cm thick and comes with a protective sheath which is disposable
• the probe is prepared by coating it with the ultrasound gel
• it is then inserted in your vagina to ‘scan’ the baby
• while this method may provide better results, you can consult your doctor if you are uncomfortable about performing this procedure

Who Can Perform NT Scan During Pregnancy?

The NT scan is basically an ultrasound sonography. Any qualified sonographer can hence perform the test successfully. It would be best to ask your gynaec for trusted recommendations of a good sonographer. While the procedure may not seem very difficult, you cannot rely on just a technician or an assistant to perform the test.

When is NT Screening Done?

The NT scan is typically performed between the 11^th and 13^th week of your pregnancy. This is because – your baby’s neck is still transparent till this point. It is not possible to perform the NT scan beyond 13 weeks and 6 days of pregnancy – this is the latest by which you may perform the NT scan.

Risks of NT Screening

The NT scan is a non-invasive method. No blood sample is required to perform this test, no needles are inserted in any part of your body, nor are you subjected to any kind of radiation. The NT scan is thus a very safe procedure. It does not directly adversely affect you or your baby in any way.

The only risks of NT screening are psychological – anxiety, nervousness, and fear on part of the mother may considerably affect the mother’s mental state. However, you must realise that: the NT scan is only going to help you and be better prepared to take care of your baby when it comes into this world. Stay positive, and talk to your gynaec and sonographer about the test, to get answers to any questions you may have.

Nuchal Translucency Measurement Chart

In normal foetuses, the thickness of the nuchal translucency measures about 2mm. Hence, a thicker nuchal translucency is typically indicative of a higher risk of Down’s Syndrome.

• Foetuses with 1.3mm NT thickness are at low risk of Down’s Syndrome
• The normal range of NT thickness is up to 2.5mm. However, nine out of ten babies with NT thickness up to 3.5mm will be normal and will NOT suffer from Down’s Syndrome. Such values for NT thickness are typically observed in foetuses between 45 and 85mm length.
• If the NT thickness is up to or more than 6mm, the foetus is at high risk of Down’s Syndrome

It is difficult, however, to pin a definite ‘cut-off’ value for the thickness of the nuchal translucency. A lot depends on the size of the foetus, the age of the mother, and other factors too. Many a time, a foetus with Down’s Syndrome also produces more nuchal fluid; it might not be possible to determine the risk factor accurately in such a case.

This is why doctors may look for another indicative sign: development of the nasal bone. It has been indicated that absence of a nasal bone suggests a high risk of Down’s Syndrome too.

How Accurate is Nuchal Translucency Test

1. As previously explained, the NT scan only predicts how likely you are to give birth to a baby with Down’s Syndrome. It is not a ‘diagnostic’ test, but merely a screening.
2. If you consider the nuchal translucency measurement figures discussed above, you will realise that the NT scan is not highly accurate.
3. If your baby is at a high or low risk of developing Down’s Syndrome according to the NT scan, you may be advised to undertake a ‘diagnostic’ test for Down’s Syndrome. It will tell you with certainty whether your baby will be affected or not.
4. The diagnostic test for Down’s Syndrome is a blood test. It involves measurement of levels of two hormones: Human chronic gonadotropin (hCG) and Pregnancy Associated Plasma Protein A (PAPP-A).
5. A normal foetus produces and releases these two hormones in the mother’s bloodstream. Level of these hormones can indicate whether your baby has Down’s Syndrome or not.
6. The NT scan on its own is about 70 to 75% accurate; however, combined with a blood test, its accuracy can increase up to 90%.

Who Should Go for Nuchal Translucency Screening?

Mothers who have previously given birth to a baby with Down’s Syndrome, are at a higher risk of doing so a second time. It is highly advisable for such women to undertake an NT scan.

The risk factor for Down’s Syndrome increases with maternal age. While 1 in 1500 babies born to women in their 20s will develop Down’s Syndrome, this chance increases when we consider women in their 40s: a 40-year old woman has a 1 in 100 chance that she might give birth to a baby with Down’s Syndrome, while a 45-year old woman has a 1 in 50 chance. Mothers are advised to go for the NT screening accordingly.

Why Should You Have Nuchal Translucency Ultrasound?

As a mother, it is understandable that you would be wary of getting any ‘tests’ done. Any test you are advised to take will get you feeling concerned about your baby’s safety first and foremost, followed by your own safety. The previous sections of the article might have assured you about the safety aspect of the NT scan. Yet the question remains: why should you have nuchal translucency ultrasound done?

The NT scan – along with identifying the risk factor for Down’s Syndrome can detect several other things:

• Risk factor for Patau Syndrome (a congenital defect where the baby is born with three copies of chromosome number 13, instead of the normal three copies.)
• Risk Factor for Edward’s Syndrome (a congenital defect where the baby is born with three copies of chromosome number 18, instead of the normal three copies.)
• Risk factor for physical conditions like congenital heart problems.

The NT scan can thus better equip you in terms of knowledge to make your choice about whether you want to continue your pregnancy or not.

NOTE: The Medical Termination of Pregnancy Act, 1971, allows a woman to medically terminate her pregnancy (i.e. have an abortion) up to the 20^th week of pregnancy, provided that continuing the pregnancy might prove fatal to her or the baby, or if there is considerable risk of the baby being born with physical and/or mental handicap that will prevent it from having a quality life. It is strongly advised that you seek legal counselling and guidance before taking any step.

In most cases, it will not be required of the mother to take such a drastic decision – nature has its own way of taking care of the situation. About 50% of foetuses that have Down’s Syndrome do not complete full term and are spontaneously naturally aborted. Hence chances of you giving birth to a baby with Down’s Syndrome are much smaller than chances of such a foetus completing full term in the first place.

However, should your pregnancy continue successfully and you give birth to a baby with Down’s Syndrome, then prenatal screening tests for Down’s Syndrome can actually help you better prepare yourself for the upbringing of such a baby. Management of Down’s Syndrome can be quite challenging, especially for parents who have no prior experience of dealing with such a baby, and it always helps to be informed beforehand.