Screening for Down syndrome

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Pregnant woman having a ultrasound scan

There are screening tests that can assess your chance of having a baby with Down syndrome.

There are also diagnostic tests that will give you a definite diagnosis of Down 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 syndrome?

Down 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. There's no such thing as a typical person with Down syndrome. Like everyone, people with Down syndrome vary a lot in appearance, personality and ability.

People with Down syndrome have learning difficulties and some have more serious difficulties than others. It's hard to tell how much a baby with Down syndrome will be affected as a child or an adult. There is currently no cure but with appropriate care, some people with Down syndrome can be supported to lead an active and independent life.  

Conditions linked with Down 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 syndrome can live into their 50s, 60s and 70s.

What causes Down 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 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 syndrome, but this isn't true. Anyone can have a baby with Down 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 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 syndrome - they tell you how likely it is that your baby might have it.

First trimester combined screening

This involves a blood test and an ultrasound scan

Women may be offered a dating scan between weeks 8 and 14 of pregnancy, and a nuchal translucency scan between 11 weeks and 13 weeks and 6 days of pregnancy (sometimes called the 12 week scan). If you choose to be screened for Down syndrome, the dating scan and the nuchal translucency scan can be carried out at the same time, (between 11 weeks and 13 weeks and 6 days of 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.

You have the blood test between weeks 9 and 12. The blood test measures the levels of two different hormones that occur naturally in your blood during pregnancy. When there’s a genetic condition the levels of these hormones are usually abnormal.

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 syndrome.

Second trimester screening 

If it has not been possible for you to have the combined screening test in early pregnancy, you will be offered a different blood test between 14 and 20 weeks.

This test is sometimes called the 'triple test' or 'maternal serum screening test' (MSS), and measures 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 syndrome.

Screening results

Some maternity services give the result as 'not at increased risk' or 'at increased risk'.

If the screening test shows the risk of the baby having Down syndrome is 'not at increased risk', this means the chance of having Down syndrome is less than 1 in 300. This does not guarantee the absence of Down syndrome.  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 syndrome.

If the result shows the risk of the baby having Down syndrome is of increased risk it does not mean the baby definitely has Down syndrome. An increased risk means you will be offered a diagnostic test.

What the screening will tell you

First trimester combined screening

About 5 out of a 100 babies tested may have an increased risk result. Most of these babies will NOT have a problem.

If the nuchal translucency test is done without the blood test:

  • About 75 out of a 100 babies who have a chromosome condition called Down syndrome will return an increased risk result. 25 out of a 100 babies with Down syndrome will be missed by this test.

If the nuchal translucency ultrasound is done together with the blood test:

  • About 80-90 out of a 100 babies who have Down syndrome will return an increased risk result. 10-20 out of a 100 babies with Down Syndrome will be missed by this test.

Second trimester screening

About 5 out of a 100 babies tested may have an increased risk result. Most of these babies will NOT have a problem.

  • About 60 out of a 100 babies who have Down syndrome will return an increased risk result.
  • 40 out of a 100 babies with Down syndrome will be missed using this test.

If this test is done at the same time as a detailed ultrasound scan, it will also identify about 95 out of a 100 babies who have spinal problems or neural tube defects.

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 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 syndrome.  

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 syndrome and other chromosomal abnormalities. The results for Down 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. 

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 syndrome at an earlier stage of pregnancy than an amniocentesis test.

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 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.

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. There are associations in every state and territory you can contact for more information and support.

Related links

 
Sources:
NHS Choices (UK) - Tests for Down's syndrome
Raising Children Network - Tests in pregnancy
NSW Health - Having a baby
WA Health - First Trimester Screening: Fetal Monitoring
Date last reviewed: 
November, 2013

12 trusted resources for screening for down syndrome

  1. Screening tests for Down syndrome

    Women's and Children's Health Network

    Screening tests can be done to work out the chance that your baby has Down syndrome. The tests are not compulsory - not all women choose to have them.

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    Last reviewed: May, 2012
  2. Screening tests for neural tube defects

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    Tests can be done to work out the chance that your baby has a neural tube defect (eg spina bifida).

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  3. 18-20 week screening pregnancy ultrasound

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    Detailed information for consumers and carers about having an 18-20 week screening pregnancy ultrasound - including what it is, how to prepare, how it is done, risks and benefits and after effects

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    Last reviewed: May, 2009
  4. Tests in pregnancy

    Raising Children Network

    Find out about tests in pregnancy – 12-week and 20-week pregnancy ultrasounds and scans, Down syndrome screening, blood tests, CVS and amniocentesis.

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    Last reviewed: December, 2013
  5. Screening for Down syndrome

    Pregnancy, Birth and Baby

    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.

    Read more
    Last reviewed: February, 2014

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