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What is an NT scan?

A nuchal translucency (NT) scan is a screening test that is done during pregnancy to measure the thickness of nuchal fold, the fold of skin at the back of a baby’s neck and to assess the amount of fluid build up in it. This test helps determine the risk of genetic problems like Down syndrome (the baby has an extra copy of the 21st chromosome, which leads to intellectual and developmental delays) in the foetus. However, NT scan cannot be used as a definite procedure to detect such problems in the baby. Other diagnostic tests like amniocentesis (a test conducted on the amniotic fluid to detect genetic disorders, chromosome abnormalities and neural tube defects) and chorionic villus sampling (a test performed on placenta cells to detect genetic conditions and congenital disabilities) are required to confirm the diagnosis of genetic conditions.

The alternative terms for NT scan are Down syndrome, nuchal translucency, nuchal fold test, prenatal genetic screening and nuchal fold scan.

  1. Why is an NT scan performed?
  2. How do you prepare for an NT scan?
  3. How is an NT scan performed?
  4. What do NT scan results mean?

NT scan is recommended in pregnant women typically between the 11th and 14th week of pregnancy. It is performed to screen the baby for Down syndrome. This scan helps screen the following genetic conditions as well:

  • Congenital heart disease (problems in the structure of baby’s heart)
  • Trisomy 13 (a condition in which the baby has an extra 13th chromosome, which causes physical abnormalities and severe intellectual disability after birth)
  • Down syndrome
  • Turner syndrome (a disorder, in which, a female baby is born with only one X chromosome)
  • Trisomy 18 (a condition, in which, an extra 18th chromosome causes severe developmental delays in baby, and the baby is born with an abnormally shaped head and clenched hands)

It is recommended to have water or other fluids before the test as a full bladder is required for the scan. Avoid urinating before the test.

The pregnant woman is asked to lift her shirt and wear a waistband. Women wearing a dress will be given a cloth to cover the bottom part of their body. Once the bladder is full, the NT scan is done in the following way:

  • The woman is asked to lie on their back or the side on the examination table.
  • A gel is applied to the belly.
  • A transducer device (a handheld device that emits ultrasound waves) is pressed on the gel and moved over the belly.
  • The transducer captures images, which are then displayed on a monitor.
  • The technologist measures the thickness of the fluid collected at the back of the baby’s neck.
  • Once the test is completed, the gel is wiped off the skin, and the woman is asked to urinate. The procedure takes about 20 minutes.

There are no risks associated with an NT scan.

NT measurements increase as the pregnancy proceeds. The higher the measurements are as compared to foetuses of the same age the higher is the risk of certain genetic disorders.

Normal results:

A normal NT scan indicates an absence of Down syndrome or other genetic disorders in the baby. The following measurements of the nuchal fold indicate a low risk of genetic disorders:

  • Measurement of up to 2 mm at 11 weeks
  • Measurement of up to 2.8 mm at 13 weeks, 6 days

Abnormal results:

Higher than normal amount of fluid at the back of baby’s neck denotes a high risk of the following conditions:

  • Congenital heart disease
  • Trisomy 13
  • Down syndrome
  • Turner syndrome
  • Trisomy 18

Other tests like amniocentesis are ordered if abnormal results are obtained on the NT scan to identify the abnormality, if any.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.

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References

  1. Michigan Medicine: University of Michigan [internet]; Nuchal Translucency Screening Test
  2. Walsh JM, D'Alton ME. Nuchal translucency. In: Copel JA, D'Alton ME, Feltovich H, et al., eds. Obstetric Imaging: Fetal Diagnosis and Care. 2nd ed. Philadelphia, PA: Elsevier; 2018:chap 45
  3. Chaveeva P, Agathokleous M, Nicolaides KH. Fetal aneuploidies. In: Coady AM, Bower S, eds. Twining's Textbook of Fetal Abnormalities. 3rd ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2015:chap 2.
  4. Michigan Medicine: University of Michigan [internet]; Nuchal Translucency Screening Test: How To Prepare
  5. MedlinePlus Medical Encyclopedia: US National Library of Medicine; Nuchal translucency test
  6. National Call center network, Health direct [internet]: Department of Health: Australian government; Nuchal translucency scan
  7. Michigan Medicine: University of Michigan [internet]; Nuchal Translucency Screening Test: How It Is Done