Screening for Down’s syndrome
All pregnant women are offered screening for Down’s syndrome. This is to assess your chance of having a baby with the condition.
The screening tests are:
- a blood test plus nuchal translucency ultrasound scan (the nuchal translucency is a collection of fluid at the back of the baby’s neck) – this is called the combined test and is offered between 11 and 14 weeks of pregnancy
- a blood test that can be offered between 14 and 20 weeks of pregnancy if you are too far along to have the combined test. This blood test is less accurate than the combined test.
If these screening tests show that you have a risk greater than one in 150 (that is, between one in two and one in 150) of having a baby with Down’s syndrome, you will be offered diagnostic tests to find out for certain if your baby has the condition. The diagnostic tests are:
- chorionic villus sampling (CVS)
What is Down’s syndrome?
A baby born with Down’s syndrome will have a learning disability. This means they will find it harder than most people to understand and to learn new things. They may have communication problems and difficulty managing some everyday tasks.
It is impossible to know what level of learning disability a baby with Down’s syndrome will have. It can vary from mild to severe.
Most children with Down’s syndrome attend mainstream primary school. People with Down’s syndrome can have a good quality of life. With support from their family and others, many people are able to get jobs and live fairly independently.
Some health problems are more common in people with Down’s syndrome, for example heart conditions and problems with the digestive system, hearing and vision. Sometimes these problems can be serious but many can be treated. With good healthcare, someone with Down’s syndrome can live to around 60 years.
People with Down’s syndrome have almond shaped eyes and distinctive facial features, but they do not all look the same. Like all children, they also inherit features from their parents.
Read more about Down’s syndrome.
What causes Down’s syndrome?
People with Down’s syndrome have an extra chromosome in their body cells. Chromosomes are tiny structures inside the cells of our bodies, and they carry the genes that determine how we develop. Most people have 23 pairs of chromosomes, while people with Down’s syndrome have an extra copy of chromosome 21. You can find out more information about chromosomes.
Down’s syndrome is not usually an inherited condition. In most cases it happens by chance. There are rare occasions when some extra chromosome 21 material can be passed on. This is called translocation. You can find out more about translocation.
Women of any age can have a baby with Down’s syndrome, although the chance of having a baby with Down’s syndrome increases with age. The older a woman is, the more likely she is to have a baby with Down’s syndrome:
- A 20-year-old woman has a one in 1,500 chance of having a pregnancy affected by Down's syndrome (this is a probability of 0.07%).
- A 30-year-old woman has a one in 900 chance of having a pregnancy affected by Down's syndrome (a probability of 0.1%).
- A 40-year-old woman has a one in 100 chance of having a baby affected by Down's syndrome (a probability of 1%).
These are background risks based on the woman’s age alone. The combined screening test may give you a higher or lower risk than this.
What does the screening test involve?
A screening test for Down’s syndrome is available between 11 and 14 weeks of pregnancy. It is called the ‘combined test’ because it combines an ultrasound scan with a blood test. The scan can be carried out at the same time as the dating scan.
If you choose to have the test, a blood sample will be taken from you. At the dating ultrasound scan, the fluid at the back of the baby’s neck is measured (known as the nuchal translucency). The information from these two tests is used to work out the risk chance of the baby having Down’s syndrome.
It is not always possible to obtain this measurement and depends on the position of the baby. If this is the case you will be offered a different blood screening test between 14 and 20 weeks.
If you are too far on in your pregnancy (more than 14 weeks) to have the combined test you will be offered the blood screening test between 14 and 20 weeks of pregnancy. This test is not quite as accurate as the combined test.
Can this screening test harm me or my baby?
The screening test cannot harm you or the baby but it is important to consider carefully whether to have this test. This test cannot tell you for certain if the baby has Down’s syndrome or not.
The test can provide information that may lead to further important decisions. For example, you may be offered diagnostic tests that can tell you for certain whether the baby has Down’s syndrome, but these tests have a risk of miscarriage.
Do I need to have screening for Down’s syndrome?
It is your choice whether or not to have the screening. You do not need to have this screening test. Some people want to find out the risk of their baby having Down’s syndrome, and some do not.
What if I decide not to have this test?
If you choose not to have the screening test for Down’s syndrome, you can still choose to have other tests.
If you choose not to have screening for Down’s syndrome it is important to understand that if you have a scan at any point during your pregnancy it could pick up physical abnormalities that could indicate Down’s syndrome. The person scanning you will always tell you if any abnormalities are found.
Getting your results
The screening test will not tell you if your baby has Down’s syndrome or not. It will tell you if you have a higher or lower risk of having a baby with Down’s syndrome.
If your screening test shows a lower risk result, you should be told within two weeks of the test being taken.
If your screening test shows a higher risk result, you should be told within three working days of the blood test result being available. This may take a little longer if your test is sent to another hospital. It may be worth asking the midwife what happens in your area, and when you can expect to get your results. You will be offered an appointment to discuss the test results and the further options you will have.
The charity Antenatal Results and Choices (ARC) offers lots of information about screening results and the options you face if you get a higher risk result. It also has a forum where you can be put in touch with people who are in the same situation as you. You can read about people’s experiences of making their decisions on the ARC website.
If the screening test shows that the chance of having a baby with Down’s syndrome is lower than one in 150, this is a ‘lower risk’ result. More than 95 out of 100 screening test results will be lower risk. A lower-risk result does not mean there is no risk at all of the baby having Down’s syndrome.
If the screening test shows that the chance of the baby having Down’s syndrome is higher than one in 150 (that is, between one in two and one in 150), this is called a ‘higher risk’ result. Fewer than one in 20 (5%) of screening test results will be higher risk. This means that of 100 women accepting screening for Down’s syndrome, fewer than five will have a higher-risk result.
A higher-risk result does not mean the baby definitely has Down’s syndrome.
Will I need further tests?
If you have a lower-risk result you will not be offered further tests for Down’s syndrome.
If you have a higher-risk result you will be offered a diagnostic test, such as amniocentesis or CVS. This will tell you for certain whether the baby has Down’s syndrome or not.
About one in 100 diagnostic tests result in a miscarriage. It is up to you whether or not to have the further tests. When trying to decide whether to have a diagnostic test, try to balance up the risk of miscarriage with how important the result will be to you.
Chorionic villus sampling (CVS): this diagnostic test is usually done between 11 and 14 weeks of pregnancy. A fine needle, usually put through the mother’s abdomen, is used to take a tiny sample of tissue from the placenta. The cells from the tissue are then tested for Down’s syndrome. Read more about CVS.
Amniocentesis: amniocentesis is done from around week 15 of pregnancy. A fine needle is passed through the mother’s abdomen into the uterus to collect a small sample of the fluid surrounding the baby. The fluid contains cells from the baby, which are tested for Down’s syndrome. Read more about amniocentesis.
If you find out your unborn baby has Down’s syndrome
A small number of women who have a diagnostic test will find out their baby has Down’s syndrome. They then have two options. Some women decide to continue with the pregnancy and prepare for their child with Down’s syndrome; others decide they do not want to continue with the pregnancy and have a termination (abortion).
If you are faced with this choice, you will get support from health professionals to help you make your decision.
Support for parents is also available from the charity Antenatal Results and Choices.
The Down's Syndrome Association has information that you might find useful.
Screening tests for you and your baby (PDF, 3.84Mb) is a booklet produced by the UK National Screening Committee.