The nuchal translucency scan
The main objective of the nuchal scan is to assess the risk of Down syndrome, although it does also check foetal anatomy and can detect some abnormalities.
How does down syndrome occur?
Down syndrome occurs when the baby receives an extra chromosome (trisomy 21). It can occur at any age but is more common in older woman. A definite diagnosis of Down syndrome can only be made by invasive testing such as amniocentesis or chorionic villous sampling (CVS).
How can you test for down syndrome?
Amniocentesis is done at around 16 weeks. A needle is passed through the mother’s abdomen, through the uterine wall and into the fluid that surrounds the baby. A small sample of this fluid is removed.
A CVS is done at around 11 weeks. A needle is passed through the uterine wall into the placenta and a small sample of the placenta removed. In both cases the sample is sent to a genetic laboratory where foetal cells are cultured and the chromosomes analysed.
Who gets tested?
In the past, risk assessment for Down syndrome was based only on maternal age and all women over the age of about 35 were offered the option of amniocentesis at 16 weeks. But this only detected 30% of Down syndrome because although the risk of Down syndrome is higher in older woman, younger woman have more babies and thus more babies with Down syndrome were born to younger people.
Risks involved when testing for Downs Syndrome
Invasive tests carry a 1% risk of miscarriage. In other words 1 in 100 healthy babies would be lost as a direct result of amniocentesis or CVS. With this in mind, other tests have been developed to asses the risk of Down syndrome, combining mother’s age, the nuchal translucency measurement and maternal blood tests.
The nuchal translucency scan and maternal blood tests
Maternal blood tests
Blood taken from the mother is used to measure 2 hormones that are secreted by the placenta. The levels of these hormones change in Down syndrome babies.
The nuchal translucency scan
During the nuchal scan the thickness of the fluid at the back of the baby’s neck is measured. All babies have some fluid here but in babies with Down syndrome the fluid thickness is increased.
The scan is usually done using the abdominal probe, which has the bonus of giving a good view of the baby. For the first time the little hands and feet are distinct and facial features can be clearly seen.
As a guideline, invasive testing is recommended when the risk of Down syndrome is 1:300 or worse. However, the decision to go ahead depends very much on personal circumstances. The perception of risk is a very individual matter and can vary enormously from patient to patient.
It is vitally important that the implications of this scan are discussed with you in detail and that you fully understand these implications before the scan is done.
Understanding first trimester screening
- The tests determine risk only; they do not detect Downs.
- The decision to do the test is an extremely personal one that depends to a large extent on what you would do if the final invasive test showed that your baby had Down syndrome. Would you terminate the pregnancy or would you continue anyway?
- Even if the test shows significantly increased risk, the probability remains that the baby is healthy. A risk of 1 in 50, for example, is very high but chances are still very much in favour of the baby being fine (i.e. 49 out of 50);
- First trimester screening is an extremely valuable tool in obstetric practice. The vast majority of screening tests will be negative and thus very reassuring. The testing can also reduce the need for invasive tests in older women.
The testing may however not be appropriate for everyone, and careful consideration should be given to the implications of a positive screening result before the test is performed.