What tests are there for Down's syndrome?
There are screening tests that can assess your chance of having a baby with Down's syndrome. There are also diagnostic tests that will give you a definite diagnosis of Down's syndrome and, sometimes, other abnormalities. Your midwife or doctor will offer you information to help you decide if you want the tests or not.
What is Down's syndrome?
Down's syndrome, also known as Trisomy 21, is caused by the presence of an extra (third) copy of the chromosome number 21. It affects around one in every 1,000 babies born. In 2011, just under 724,000 babies were born in England and Wales, and 725 of these were born with Down’s syndrome. There's no such thing as a typical person with Down's syndrome. Like everyone, people with Down's syndrome vary a lot in appearance, personality and ability.
People with Down's syndrome have learning difficulties and some have more serious difficulties than others. It's hard to tell how much a baby with Down's syndrome will be affected as a child or an adult. There is currently no cure but with appropriate care, some people with Down's syndrome can be supported to lead an active and independent life.
Conditions linked with Down’s syndrome include heart problems and reduced hearing and vision. Many of these problems can be treated, and frequent health checks can ensure that any problems are detected as early as possible. People with Down's syndrome can live into their 50s, 60s and 70s.
What causes Down’s syndrome?
Inside the cells of our bodies are tiny structures called chromosomes. Chromosomes carry the genes that determine how we develop. Most people have 23 pairs of chromosomes in each of their cells. When our bodies produce the sperm and egg cells needed to create babies, the chromosome pairs divide and rearrange themselves. Sometimes these pairs don't divide correctly, and this causes the baby’s cells to have an extra copy of chromosome number 21. This causes Down’s syndrome.
The extra chromosome can't be removed from cells, so there's no cure for the condition. The chromosomes divide incorrectly by accident, not because of anything the parents have done. Some people think that only older women can have a baby with Down’s syndrome, but this isn't true. Anyone can have a baby with Down’s syndrome, but the risk increases with age. The older a woman is, the more likely she is to have a baby with the condition.
Screening
Screening for Down's syndrome and other genetic disorders is offered to all pregnant women. There are different ways of carrying out screening tests.
Screening tests don’t tell you for definite if your baby has Down’s syndrome – they tell you how likely it is that your baby might have it.
Combined screening
This involves a blood test and an ultrasound scan. All women should be offered a dating scan between weeks 8 and 14 of pregnancy, and a nuchal translucency scan between 11 weeks, 2 days and 14 weeks, 1 day of pregnancy. If you choose to be screened for Down's syndrome, the dating scan and the nuchal translucency scan can be carried out at the same time, between 11 weeks, 2 days and 14 weeks, 1 day of pregnancy.
The blood test measures two proteins associated with pregnancy. At the ultrasound scan appointment, the sonographer measures the thickness of the nuchal translucency (a pocket of fluid) at the back of your baby's neck. The information from the blood test is combined with your age and the nuchal translucency measurement and used to work out your individual chance of having a baby with Down's syndrome.
Screening later in pregnancy
If it has not been possible for you to have the combined screening test in early pregnancy, you will be offered a blood test between 14 weeks, 2 days and 20 weeks.
This test is sometimes called the quadruple test, and measures four proteins associated with pregnancy. This information is combined with your age and used to work out your individual chance of having a baby with Down's syndrome.
Screening results
Some maternity services give the result as "lower risk/screen negative" or "higher risk/screen positive". These are explained below.
Lower-risk result
If the screening test shows the risk of the baby having Down's syndrome is lower than the recommended national cut-off level for risk (1 in 150), this is known as having a lower-risk result. A lower-risk result means that you will not be offered a diagnostic test, but it doesn't mean that there is no risk at all of the baby having Down's syndrome.
Higher-risk result
If the result shows the risk of the baby having Down's syndrome is greater than the recommended national cut-off level for risk (1 in 150), this is known as an increased risk, or higher-risk result. An increased risk means you will be offered a diagnostic test but it does not mean that your baby definitely has the condition.
Diagnostic tests
The diagnostic test will be either chorionic villus sampling (CVS) or amniocentesis, which will give you a definite answer as to whether your baby has Down's syndrome or not. Your midwife or doctor will explain the result to you and help you to decide whether you want to have further testing.
Amniocentesis
After counselling, amniocentesis may be offered from the time you are 15 weeks of pregnancy if you have a higher-risk result for Down's syndrome.
What happens at amniocentesis
Using ultrasound as a guide, a fine needle is passed through the wall of the abdomen (tummy) into the amniotic fluid that surrounds the baby inside the uterus (womb). The cells within the fluid contain the same chromosomes as the baby. A small sample of this fluid is withdrawn and sent to a laboratory for testing. A local anaesthetic is not usually used, and most women feel only mild discomfort. The test takes 5-10 minutes.
The fluid will be tested for Down's syndrome and other chromosomal abnormalities. The results for Down's syndrome and other common chromosomal abnormalities should be available within three working days. However, if all the chromosomes have to be looked at, it can take up to three weeks. This test will reveal your baby's sex, so decide if you want to be told at the same time as you receive your diagnostic test results whether your baby is a boy or a girl.
The risks
Amniocentesis has a 0.5%-1% risk of causing a miscarriage. At most, one test in a hundred will result in pregnancy loss. When deciding whether or not to have this test, try to balance the risk of miscarriage with how important the result will be to you.
Chorionic villus sampling (CVS)
CVS can be carried out from around 11 weeks of pregnancy. It can tell you whether or not your baby has Down's syndrome at an earlier stage of pregnancy than an amniocentesis test.
What happens in CVS?
Using ultrasound as a guide, a fine needle is passed through the abdomen (tummy) into the placenta inside the uterus (womb). Occasionally, the position of the placenta means that the procedure cannot be done through your abdomen, and a fine tube is passed through the vagina and cervix into the womb instead.
A tiny piece of the developing placenta, known as chorionic villous tissue, is withdrawn. The chromosomes in chorionic villous cells are examined. The test may be a little uncomfortable – a local anaesthetic is usually used to reduce discomfort. The test takes 5-10 minutes.
As with amniocentesis, a rapid result for Down's syndrome and other common chromosomal abnormlities can be obtained within three working days, but if all the chromosomes are checked the results can take up to three weeks.
The risks
CVS has a 1% risk of miscarriage.
If a test detects an abnormality
It's always difficult when you're told that something is wrong with your baby, especially if you're faced with a painful decision about the future of your pregnancy. Your midwife or doctor will make sure you see the appropriate health professionals to help you get all the information and support you need so you can make the right choices for you and your family.
The organisation Antenatal Results and Choices helps parents with all the issues around antenatal testing and its implications. They can give you more information or put you in touch with parents who decided to continue with a pregnancy in which an abnormality had been detected, or those who decided to have a termination of pregnancy.
Healthtalk online has interviews with women talking about their experiences of antenatal screening and the decisions they made.
The Down's Syndrome Association has some booklets that you might find useful, including:
The NHS Screening Programme website has more information about foetal anomaly screening.
