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Everything You Need to Know About NT NB Scan

  • Published on September 06, 2022
Everything You Need to Know About NT NB Scan

If you are newly pregnant, your doctor may advise you to undergo some screening tests. Irrespective of age, all pregnant women are asked to undergo these tests to ensure the good health of the baby and the expecting mother.

A nuchal or nuchal translucency (NT) scan is one such prenatal screening scan to check for any chromosomal abnormalities in the growing foetus. The nasal bone (NB) scan is part of the NT scan.


What is the NT NB Scan?

An NT scan measures the fluid-filled space behind the baby’s neck called the nuchal translucency. Once the doctor has the exact measurements, they can estimate if your baby has a risk of having any chromosomal abnormalities like Down syndrome.

The test must be performed during the first trimester of the pregnancy as the clear space at the back of the baby’s neck starts disappearing after 15 weeks.

During the NT NB scan, along with measuring the nuchal translucency, the thickness of the nuchal fold is also measured. Besides that, the test checks whether the baby has developed a nasal bone. The absence of the nasal bone and a very thick nuchal fold indicates Down syndrome.

The NT scan also checks for other congenital disabilities like Edwards’ syndrome, Patau syndrome, skeletal defects, heart defects, etc.


How is the NT NB Scan Performed?

For the NT NB ultrasound scan, the health practitioner will begin by taking an abdominal ultrasound. The high-frequency sound waves of the ultrasound test will produce an image of the inside of your body.

From this image, the doctor will then measure the nuchal translucency. Other factors like the mother’s age, delivery due date, etc., are also taken into consideration to calculate the risk of any abnormality the foetus might have.

The scan will be completed in less than 30 minutes, during which you will be expected to lie on your back on the examination table. This will allow the technician to move the ultrasound stick over your abdomen easily.

The NT NB scan can also be performed transvaginally. For this method, a well-lubricated ultrasound probe will be inserted into your vagina to scan your uterus. The doctor will then use the resultant photo scan to measure the nuchal translucency and check the presence of the nasal bone.

A vaginal NT NB scan might be a little uncomfortable but not painful. It is performed by a trained professional and will be completed in less than 30 minutes.

Furthermore, both scanning methods do not harm the health of the growing baby or the expecting mother.


How to Prepare for an NT NB Scan?

You need not follow any extra measures or precautions before appearing for the NT NB scan. If you have any previous medical history which requires special attention, your doctor will advise you accordingly.

You might or might not get results the same day. Your doctor will discuss the results with you as soon as they get them.

Do not stress yourself excessively while waiting for the result. For most expecting mothers, the NT NB scan is done as a safety measure.


Advantages of the NT NB Scan

It is highly advisable to get an NT NB scan done, along with other prenatal screening tests. It will allow you to get an accurate picture of the health of your developing baby through the following:

  • Detecting chromosomal abnormalities like down syndrome
  • Detecting structural abnormalities like spina bifida
  • Guessing a more accurate delivery date
  • Early diagnosis of any pregnancy failure risks
  • Diagnosis of multiple foetuses (if any)


Accuracy of the NT NB Scan in Pregnancy

NT and NB scans carry an accuracy rate of 70%. The NT scan misses detection of around 30% of growing babies with down syndrome.

The accuracy of NT NB scans can be greatly improved when combined with other prenatal screening tests during the first trimester.


NT NB Scan Results

Getting the NT NB scan done after the first trimester will not give accurate results.

At 14 weeks, the nuchal space is not completely closed but becomes smaller. Hence, when the NT scan is done at 14 weeks, a baby with chromosomal conditions will also show normal results.

According to the average first-trimester growth, a nuchal translucency measurement of less than 3.5 mm is considered normal. A baby with a nuchal space measurement of 6 mm or higher has a very high chance of having chromosomal abnormalities like Down syndrome as well as other heart defects.


What are the Alternatives?

Generally, a NT/NB scan is recommended in the first trimester to detect any congenital abnormalities. The alternative to the NT scan is non-invasive prenatal testing (NIPT), it is also known as cell-free DNA testing (cfDNA). 


In Conclusion

Due to changing lifestyles and a variety of other factors, congenital disabilities in growing babies are becoming increasingly common.

If you or your loved one are pregnant, you must schedule prenatal screening tests for the safety of the expecting mother and the baby.

To avail of the best screening tests, procedures, and treatment, visit your nearest Birla Fertility and IVF Clinic or book an appointment with Dr Rachita Munjal.



1. What are NT and NB scans in pregnancy?

During the first trimester, there is a fluid-filled space behind the foetus’ neck called the nuchal translucency. An NT scan is done to measure the nuchal space and estimate the risk of any chromosomal abnormalities the baby might have. The NT scan also checks if the baby has a nasal bone or not.


2. What is a normal NT NB scan?

A normal NT scan result (performed during the first trimester) will have a measurement of less than 3.5 mm. Anything wider than 3.5 mm indicates the risk of the baby having chromosomal or structural abnormalities.


3. In which week is the NT NB scan done?

The NT NB scan is performed during the first trimester (first 12 weeks of your pregnancy). The test cannot be performed after the first trimester as the baby grows larger, filling the nuchal space.


4. What happens if NT scan is not normal?

The normal NT scan measurements range from 1.6 mm to 2.4 mm. If the NT is abnormal, it suggests the fetus has chromosomal abnormalities. 

Written by:
Dr Rachita Munjal

Dr Rachita Munjal

Consultant – Birla Fertility and IVF
Dr Rachita Munjal completed her MBBS from Dr D. Y. Patil Medical College, Hospital & Research Centre, Pune and MS (Obstetrics and gynaecology) from Dr. D.Y Patil Vidyapeeth Pune. She has also completed her DAGE (Diploma in Advanced Gynae Endoscopy) from Germany and MRCOG-I (Royal College of Obstetricians & Gynaecologists, UK) Membership Examination.

Over 14 years of experience

Gurgaon, Haryana

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