Down's syndrome testing in pregnancy
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Down’s syndrome is a genetic disorder where the affected person carries an extra chromosome 21 in each cell. People with Down’s syndrome have varying degrees of learning difficulty and a characteristic appearance. They may also have heart and lung problems. Down’s syndrome is sometimes called Trisomy 21.
A variety of foetal screening tests are available now that can be carried out in the first and/or second trimesters of pregnancy. These are based on either:
- measuring the nuchal translucency, or
- measuring the levels of naturally occurring pregnancy hormones in the mother’s blood.
If the results of a screening text show that there is a relatively high likelihood of the baby having Down’s syndrome, you will be offered a diagnostic test, such as chorionic villus sampling or amniocentisis, which will give you a definite answer.
The nationally recommended Down’s syndrome screening process is the combined screening test which measures the nuchal translucency (see below) and two pregnancy hormones called Papp-A and HcG. This information is combined with the mother’s age and the gestation of the baby to give a result. It is performed between 11 and 13 weeks of pregnancy. The combined screening test is not yet available in every area.
Recent developments in antenatal investigations, known as non-invasive prenatal testing, mean that a sample of mum's blood taken early in pregnancy can reveal a range of abnormalities. Some abnormalities may still need a diagnostic confirmation after testing, using more invasive methods.
Where the combined test is not available, or for women who book after 13 weeks, hospitals will offer a blood test to screen for Down’s syndrome between 16 and 19 weeks. This will either be:
- the triple test (three hormones are measured: Hcg, afp, uE3) or
- the quadruple test (four hormones are measured: afp, Hcg, uE3 and inhibin).
An integrated screening test is available privately. This test combines information from the combined screening test and the quadruple test to provide one overall result. Your midwife will be able to give you details of the screening services that are available privately near you if you ask for them.
Depending on the screening test you are offered, a combination of the following tests for Down’s syndrome will be used along with your age and the gestation of the baby to calculate your result.
The nuchal translucency scan is an ultrasound scan which measures the pocket of fluid at the back of the baby’s neck. An increased amount of fluid may mean the baby has an increased chance of having Down’s syndrome. The measurement is combined with a blood test and your age to estimate your risk.
The level of a number of different hormones may be measured in the blood test part of Down’s Syndrome screening.
Detection rates for Down’s syndrome screening vary between hospitals. The screening test you are offered should have a detection rate of at least 75%, with a false positive rate of less than 3%. The false positive rate is the percentage of women who are given a ‘screen positive’ result but whose baby does NOT have Down’s syndrome.
Understanding your result
You will be given your Down’s syndrome screening result as a statistical chance, e.g. 1 in 1000. In most hospitals, if your result is higher than 1 in 150 this will often be reported to you using the terms ‘screen positive’ or ‘high risk’ and you will be offered a diagnostic test such as CVS or amniocentesis.
If your result is a lower chance than 1 in 150, it will most often be reported as ‘low risk’ and no further testing will be offered. This 1 in 150 figure is sometimes referred to as the ‘cut off’ number.
However, a risk of 1 in 100 is very different from a risk of 1 in 4, although both results are described as ‘screen positive’ or ‘high risk’. A 1 in 100 result means that you have a 1% risk of your baby having Down’s syndrome and a 99% chance that your baby will not have the condition. But with a risk of 1 in 4 you would have a 25% chance of your baby having Down’s.
If screening shows you have an increased chance of having a baby with Down’s syndrome, you will be offered one of two diagnostic tests. These are called CVS or amniocentesis.
- Chorionic villus sampling is a diagnostic test which is usually performed between 11 and 13 weeks of pregnancy to find out whether your baby has Down’s Syndrome or another genetic abnormality. A result for Down’s syndrome can usually be given to you after three working days.
- Amniocentesis is a diagnostic test usually carried out after 15 weeks of pregnancy. You can usually get a result within three working days.
More information on amniocentesis, CVS, and the risk associated with these procedures is given in Foetal Screening in Pregnancy.
There is also ongoing research to evaluate non-invasive prenatal diagnosis (NIPD) or testing (NIPT). These tests are non-invasive because they don’t involve ‘invading’ the womb or baby.
The main advantages of NIPD are that it can be carried out much earlier in pregnancy compared to current tests and that the procedure does not carry any risk of miscarriage. The test only requires a blood test from mum, rather than having to take a sample directly from the womb as in other types of prenatal diagnosis, such as chorionic villus sampling (CVS) or amniocentesis described above.
NIPT for Down’s syndrome is not widely available on the NHS but there are private providers (you may have heard of the Harmony Test, for example). It is possible that in the future NIPT could replace today’s screening tests or diagnostic tests. However, it is being evaluated as an additional step in the existing screening process. There are false positive results and it is likely that invasive testing will be required to confirm a positive NIPT result for the foreseeable future.
The Down’s Syndrome Association offers information and resources for parents of children with Down's syndrome and parents expecting a baby with Down's syndrome. Resources include a booklet to download called 'Continuing Pregnancy' as well as FAQs and a helpline open Mon to Fri, 10am–4pm: 0845 230 0372.
NCT's helpline offers practical and emotional support in all areas of pregnancy, birth and early parenthood: 0300 330 0700. We also offer antenatal courses which are a great way to find out more about birth, labour and life with a new baby.
The NHS/Down's Syndrome Association booklet Testing for Down's Syndrome in Pregnancy gives information about Down’s syndrome and about testing for it, so you can decide whether to have the tests.
Continuing Pregnancy with a diagnosis of Down's Syndrome - A guide for parents is a booklet for those who have had a pre-natal diagnosis of Down's syndrome and are continuing pregnancy, including personal stories from parents as well as information about the condition.
Healthtalkonline includes a number of case studies of antenatal testing for Down’s syndrome, including both those who did and did not continue the pregnancy.
Antenatal Results and Choices is a national charity that provides information to expectant and bereaved parents throughout and after the antenatal screening and testing process. Telephone helpline: 0845 077 2290