Should I be screened for Down’s syndrome? What is Down’s syndrome?

Screening for Down’s syndrome – what is it and should I get screened?

7 things to know about Down’s syndrome screening in pregnancy:

1. Down’s syndrome is a chromosomal problem that causes learning difficulties as well as physical problems. 
2. All women are offered screening tests in the NHS. 
3. These tests will only give you risk of having a Down’s baby. They do not give a diagnosis.
4. The result is reported as either higher or lower risk. Lower risk means less than 1 in 150 chance of having a baby with Down’s syndrome.
5. A higher risk result will mean you will be offered additional tests, including the option of either an amniocentesis or CVS. These tests are associated with a small risk of miscarriage.
6. An alternative is to opt for a further advanced screening blood test, known as non-invasive prenatal testing (NIPT) which can avoid the need to have an amniocentesis or CVS. 
7. NIPT is also available privately and can also given an indication of the sex of the baby.

What is Down’s syndrome and how does it occur?

Down’s syndrome is a condition where the baby inherits an extra chromosome. Chromosomes are like the filing cabinets that carry all the genetic information that the body needs, so Down’s syndrome babies and children have extra genetic material which causes them to have lots of health problems. Down’s syndrome is the most common of all the different chromosome problems and overall, it happens in about 1 in 600 babies. The older the mother gets, the higher the chance that Down’s syndrome occurs. Children with it can have difficulties with varying severity. All the children have a characteristic appearance, and they all have learning difficulties. Mostly, they are not able to go to mainstream school. They can also have physical problems including heart defects among others. There is a very high risk of dementia developing in adulthood and a high risk for cancer too. During pregnancy, there is a higher risk of stillbirth.

The screening test

Because Down’s syndrome is such a serious condition, there is a test to screen for the disorder offered to all pregnant women. This is an optional test, you do not have to have the test. For some women, this is a case of knowing what to expect so they can prepare, for others, they may not want to continue the pregnancy and would choose to end it (abortion or termination of pregnancy). 

It is important to understand that the test offered is a screening test and not a diagnostic test. This means it does not tell you whether the baby has Down’s or not, it only tells you the risk that your baby has Down’s syndrome. This is the biggest difficulty with the test as it can cause a lot of anxiety. It also means that the test misses a proportion of the babies with Down’s syndrome, especially in younger women. Overall, the NHS screening test detects around 80-85% of babies with Down’s syndrome, meaning that 15-20% will be missed.

When would the test happen?

The tests that the NHS offer are either the first trimester combined test, which is a combination of a measurement from the first ultrasound scan (nuchal translucency, which is fluid on the back of the baby’s neck) and a blood test for two hormones, or the Quad test, which is a measure of 4 hormones. The First trimester test is done between 11 and 14 weeks, and the quad test is done later, between 14 and 20 weeks. You will be offered the quad test if you book too late for the first trimester test, or if it was not possible to take the nuchal measurement on the ultrasound scan (this can sometimes happen if the baby is in a difficult position, or if you have a raised BMI and the scan is difficult). The first trimester test also screens for two other chromosome problems, called Edwards and Patau’s syndrome. The quad test, however, only screens for Down’s syndrome.

Risk level and what this means

You will receive a result as either ‘higher risk’ or ‘lower risk’, together with an actual number indicating that risk. This risk is calculated taking into account your age and the number of weeks of the pregnancy. A ‘lower risk’ result is one with a risk of less than 1 in 150. So for example 1in 200, 1 in 5000. If the risk is higher than 1 in 150, this will be called a ‘higher risk’ result, so for example 1 in 2 or 1 in 100. 

It is important to understand risk and this can be difficult. If you have a risk of 1 in 100, this means that out every 100 babies just like yours, then 1 would be expected to have Down’s syndrome, so the risk is 1%. If you have a risk of 1 in 2, then out of every 2 babies just like yours, 1 would be expected to have Down’s syndrome, so the risk is 50% or 50:50. 

If you have a lower risk result, then there will be no further offer of testing, and the scan of baby at 20 weeks will happen as usual.

What if I receive a higher risk result?

If you have a higher risk result, then the midwife will discuss the options with you. These are:

1. Do nothing. If you choose this option, then you will just wait and see until after the baby is born. Whilst a 20 week scan can be reassuring, it does not reduce the risk of the baby having Down’s syndrome as many babies with Down’s syndrome look normal on scan.
2. Opt for a further, advanced screening test, known as non-invasive prenatal testing (NIPT). This is another blood test, and it looks for cells from the baby in your circulation. The test then looks for evidence of Down’s syndrome in these cells. This is still an advanced screening test as it can rarely be wrong, so it still reports a risk rather than an actual yes or no. However, the test results are much more polarised, in that the result with either be very low, or very high. The high risk results are reported as ‘a greater than 99% chance affected’. If you get a high-risk result, you would still be recommended to have an invasive test to confirm before taking permanent decisions about the pregnancy. This test is also available privately (there are many manufacturers, including IONA, Harmony). Some of the private tests also offer the option to test for other problems. However, be careful with these other tests as they can sometimes be inaccurate and interpreting them can be difficult. This test also gives the option of assessing the sex of the baby. However, rarely, the NIPT can get the sex of the baby wrong but you would not always know this until the 20 week scan or even after birth. If the NIPT does get the sex wrong, if does not mean that the Down’s part of the test is also likely to be wrong as they are different tests on the same sample. It is important not to lose confidence in the Down’s test if an error is made with the sexing.
3. Opt for invasive testing by referring you to a fetal medicine doctor. This is a diagnostic test and will give you a definite answer. This is a needle into the uterus to collect cells either from the placenta (CVS, chorionic villus sampling from 11 to 14 weeks) or from the baby’s fluid (amniocentesis done after 15 weeks). Both these tests have a risk of miscarriage which is present regardless of whether the pregnancy is affected or not. The risk of miscarriage is 1% for CVS and 0.5% for amniocentesis. The amniocentesis is usually a quicker and less painful test. The CVS uses a bigger needle, needs local anaesthetic and takes a bit longer. Sometimes it is not possible to do this test if the placenta cannot be reached. Both tests produce a result within a couple of working days for Down’s, Edward’s and Patau’s. Sometimes, the test is higher risk because the baby’s nuchal measurement is much higher than expected. These babies can have additional risks and this will be discussed with you by the doctor in the fetal medicine appointment.