Nuchal translucency scan

What is a nuchal translucency scan?

Nuchal Translucency is a combined screening test, which involves an ultrasound scan and a blood test. This gives the most accurate result for assessing the risk of Down’s syndrome in a foetus during pregnancy. A Nuchal Translucency (NT) scan is a screening test that is carried out to assess whether a baby is likely to have Down’s syndrome or other chromosomal abnormalities. The NT scan cannot diagnose whether a baby has Down’s, it only estimates the risk. However, the scan can help a woman decide whether or not to have a diagnostic test.

What is nuchal translucency?

Nuchal Translucency is the collection of fluid under the skin at the back of the baby’s neck. The nuchal is measured using ultrasound when the foetus is between 11 weeks and 13 weeks plus six days gestation. All foetuses will have some fluid; those with Down’s syndrome have an increased amount.

What is Down’s syndrome?

Down’s Syndrome is a genetic condition caused by the presence of an extra chromosome in the body’s cells. It is not a disease and it is not a hereditary condition. Down’s syndrome occurs by chance during conception. Anyone can have a baby with Down’s syndrome; however the risk increases with age. The chance of having a baby with the condition is 1 in 1500 for women 20 years of age, 1 in 900 for women 30 years old and 1 in 100 for women who are 40 years old. 1 in 1000 babies is born with Down’s syndrome. Six hundred babies with Down’s syndrome are born in the UK every year.

Testing for Down’s syndrome

Expectant couples have the choice to test their unborn baby for Down’s syndrome. A screening test will give an indication of whether or not their baby has an increased risk of Down’s syndrome and other chromosomal abnormalities such as Edward’s and Patau syndromes. The screening test does not carry a risk of miscarriage as it is non-invasive but it does not give a diagnostic answer. Diagnostic tests such as chorionic villus sampling and amniocentesis will give a definitive answer but both carry the risk of miscarriage. 

The combined screening test: 

The combined screening test looks at both the nuchal translucency ultrasound scan and a blood test from the mother to work out a risk figure. A Nuchal Translucency scan (NT) measures the amount of fluid lying under the skin at the back of the baby’s neck. Babies with Down’s syndrome, heart and other chromosomal abnormalities have an increased amount of fluid here. The scan will also provide you with additional information on multiple pregnancy, accurately date your pregnancy, and will also exclude major abnormalities and miscarriage at this stage of the pregnancy.

The blood test measures the amount of a hormone and a protein that are found naturally in the mother’s blood. These substances have passed from the baby to the mother. A computer programme uses the nuchal scan measurement, the size of the baby, the mother’s age and the blood results to work out a risk for Down’s syndrome for the baby. This combined test allows the screening process to be more accurate. 

When is the best time to have a nuchal translucency scan?

The best time to perform this test is between 11 and 14 weeks of gestation. Women who are unsure of their dates are recommended to have a dating scan before the screening test.

Who performs the scan?

The nuchal translucency scan is performed by an experienced trained ultrasonographer who is certified for nuchal translucency scanning.

What do the results mean?

The results provide you with a risk factor for the chance of your baby having Down’s syndrome. The information you receive from the combined screening test will help you make a decision on whether to have no further tests carried out or whether you decide to opt for a further invasive diagnostic test. It is recommended that you discuss the type of invasive tests with your midwife/doctor before proceeding. As a guide, if the combined test shows the risk of the baby having Down’s syndrome is relatively high (e.g. one in 100) an invasive test is usually offered.

What to expect during a nuchal scan

Before a nuchal scan you will need to drink a pint of water so that you attend with a full bladder. The scan is performed on your tummy. If your baby is in a difficult position an internal scan may be needed.

During the scan the pregnancy will be dated, measured and assessed for major physical abnormalities. If the baby is between 11 and 14 weeks a blood sample will be taken. If the results show an increased risk you will be offered a consultation with a consultant obstetrician to discuss the results and the next steps. To have a combined nuchal screening test, you can self refer or request a referral through your GP or midwife.

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