The nuchal translucency is a name for the collection of fluid behind the neck of a foetus that can be seen on ultrasounds in early pregnancy. The fluid is present in all foetuses; however an increased collection indicates chromosome abnormalities present in conditions such as Down’s syndrome. It is called a 'translucency' because on an ultrasound this appears as a black space beneath the fetal skin.
A nuchal translucency scan is an optional ultrasound offered to all pregnant women during their first trimester. The scan is carried out between week 11 - 13 of pregnancy and is used to screen for Down’s syndrome, or other chromosomal conditions by detecting and measuring the amount of nuchal translucency in the foetus.
An increased amount of nuchal translucency on the scan indicates an increased collection may a be a sign of a chromosomal abnormality, however this is just an indication rather than a confirmed diagnosis.
The NT scan can be combined with a blood test, known as the first trimester combined screening, to get a more accurate prediction. The blood test will measure the presence of 2 hormones hCG and PAPP-A, that are produced by the foetus and passed into the mother’s bloodstream. Low-levels of PAPP-A in early pregnancy may be associated with the placenta not working as well as it should do (some studies suggest). This can lead to some babies not growing as well as expected which would mean you would be offered extra scans. Low- PAPP-A can also be associated with Down's syndrome; however it is possible to have low levels of PAPP-A and a low chance screening result; this will be explained to you by your screening midwife.
The blood test, combined with the results of NT screening and the mother's age, provide a more accurate assessment - without the blood test, screening is 75% accurate for predicting Down’s syndrome, with the blood test accuracy increases to 85%.
What happens during a Nuchal Translucency scan?
You’ll be referred by your doctor for you NT scan, which will normally take place at a hospital and be carried out by a sonographer.
The scan usually takes about half an hour. You’ll be asked to lie down on your back, the sonographer will then apply gel across your lower abdomen and move a handheld device called a transducer across the gel. The scan shouldn’t be painful but you may feel some pressure as the sonographer presses on your abdomen to get a clear picture.
Occasionally it can be difficult for the sonographer to get a clear scan from an abdominal ultrasound, in which case a vaginal ultrasound will be offered. This is when a small transducer will be inserted into the vagina to get a closer look at the foetus and clearer images. This method can be a little uncomfortable but is safe for the baby.
What are the risks of a Nuchal Translucency scan?
There are no known risks to mother or foetus when having a nuchal translucency scan and no after effects of an abdominal or transvaginal ultrasound.
However, it is important to be aware of the implications of taking the test and the decisions to consider based on results. If the result of your scan is high-risk you could be offered further diagnostic tests that can tell you for certain whether your baby has these conditions, however these more invasive tests do have a risk of miscarriage.
Getting the results of my NT scan
The time it takes for you to receive the results of your NT scan can depend on various factors including the complexity or urgency of results, but you should usually receive them within 2 weeks.
When you receive the results, it is important to understand what a normal NT measurement is. For a baby that is between 45 mm and 84 mm in size, an NT of less than 3.5mm is considered normal. An NT less than 1.3 mm is considered to be low-chance and an NT of 6 is considered high chance for Down’s syndrome and other potential chromosomal abnormalities.
What if the results show my baby has a high chance of Down's syndrome?
If the results show a high chance of Down’s syndrome it’s important to remember the test is not 100% accurate and it’s not known for certain whether your baby has the condition.
An advantage of the test being taken early in the pregnancy is that it gives you time to carefully consider your next steps and get the support and advice you need.
You’ll have the choice to undergo further testing such as Chorionic villus sampling (CVS) or Amniocentesis, to find out for certain if your baby has chromosomal abnormalities. These tests do carry the risk of miscarriage so it can be a difficult decision to make and one not to be done in a hurry.
As an alternative, non-invasive prenatal testing (NIPT) is just becoming available in some NHS trusts or privately and offers a safer low risk option to identify conditions such as Down’s Syndrome.