Down's syndrome - Diagnosis 

Diagnosing Down's syndrome 

Antenatal screening

Antenatal screening helps you manage your pregnancy better and lets you know how your baby is progressing. Two mums talk about what screening involves.

Reliability of screening

As with most medical tests, the tests that are used to screen Down’s syndrome have a reasonably high level of accuracy, but they are not completely accurate.

Two main mistakes can be made during any medical test:

  • a false-positive result, where a test produces a positive result, such as a child having Down’s syndrome, but the result is negative in reality, i.e. the child does not actually have Down’s syndrome
  • a false-negative result, where a test gives an ‘all-clear’ by indicating that no problems have been found, but in reality the test has failed to detect a problem, such as the child having Down’s syndrome

The false-positive and false-negative rates of the various tests used in screening are listed below.

  • Blood tests - have a false-positive rate of 3-5% (which means that three to five children out of 100 who are tested will be diagnosed as having Down’s syndrome when they do not). The false-negative rate is 35% (which means that for every 100 cases where a child has Down’s syndrome, the diagnosis will be missed in 35 children).
  • Nuchal translucency ultrasound scan has a false-positive rate of 3.5-5%, and a false-negative rate of 30%.
  • Chorionic villus sampling (CVS) has a false-positive rate of 1-2%, and a false-negative rate of 2%.
  • Amniocentesis has a false-positive rate of 0.1-0.6%, and a false-negative rate of 0.6%.

No single test is entirely accurate, which is why a combination of different tests can be used for screening. However, the more invasive tests, such as CVS and amniocentesis, run a small risk of causing a miscarriage (1 in 100).

Another important factor is that the false-positive rate of a test is usually the most important factor in assessing its reliability. This is because it is much more unlikely that a child has a rare condition, such as Down’s syndrome, which means that a false-positive result is more likely.

Carefully discuss with your GP and antenatal care team the risks involved with testing, and the implication of the results.

Antenatal screening

All pregnant women, no matter what their age, should be given the choice to have screening for genetic conditions such as Down’s syndrome.

Antenatal screening is a way of assessing whether your unborn baby could develop, or has developed, an abnormality during your pregnancy.

Antenatal screening cannot diagnose conditions such as Down’s syndrome, but it can determine the likelihood of your baby developing the syndrome.

If the risk of Down’s syndrome (or any other condition) is shown to be high, further testing can be arranged to confirm whether or not your baby has the condition. The test that is used for Down’s syndrome is a combined blood test and an ultrasound scan, which is known as the ‘combined test’.

Blood test

During a blood test, a sample of your blood will be taken and tested to check the levels of certain proteins and hormones. If your blood contains abnormal levels of these substances, you may have an increased risk of having a baby with Down’s syndrome.

Nuchal translucency

You will have a number of ultrasound scans throughout your pregnancy to check the development of your baby. An ultrasound scan is a painless procedure that uses high-frequency sound waves to produce a picture of the inside of your body (in this case, your womb).

In order to screen for Down’s syndrome, you will have a special type of ultrasound scan known as nuchal translucency. This type of ultrasound works in the same way as a normal ultrasound scan, but it focuses on measuring the space between the spine and the nape (the back of the baby’s neck).

All babies tend to collect fluid behind the neck. However, babies with Down’s syndrome usually have more fluid in their neck than normal. Measuring the thickness of fluid will help to determine whether your baby is likely to have Down’s syndrome.

Timing

You should ideally have antenatal screening for Down’s syndrome by the end of your first trimester (13 weeks and six days). Most women are screened between 11 to 13 weeks. However, if necessary, it is possible to have screening up to 20 weeks into your pregnancy, although you may also need more blood tests. 

For more information about antenatal screening, see the Health A-Z topic about Antenatal appointments and the Pregnancy care planner.

Diagnostic tests

If the results of your antenatal screening or ultrasound scan suggest that your baby may be at an increased risk of a particular health problem or condition, such as Down’s syndrome, you may be advised to have further tests.

There are two types of test that can help to diagnose potential health problems or conditions while your baby is still in the womb. These are:

  • chorionic villus sampling (CVS)
  • amniocentesis

It is important to be aware that both of these diagnostic tests can, in uncommon cases, result in complications. Your GP or midwife can discuss all of your options with you in detail.

Chorionic villus sampling (CVS)

Chorionic villus sampling (CVS) involves taking a small sample of the placenta for closer examination. The placenta is the organ in which the foetus develops and is protected and nourished.

CVS can be performed after 10 weeks of pregnancy. The sample is taken either by passing a thin needle through the wall of your abdomen (tummy), or by passing a small tube through your vagina and the neck of your womb (cervix).

The needle or tube is guided into the correct position using an ultrasound scan. After the sample has been taken, it will be sent to a laboratory for testing. The results will show whether your baby has Down’s syndrome.

Possible complications of CVS can include:

  • infection
  • heavy bleeding
  • miscarriage

It is estimated that approximately 1 in every 100 woman will have a miscarriage after having CVS.

See the Health A-Z topic about Chorionic villus sampling for more information.

Amniocentesis

Amniocentesis involves taking a small sample of amniotic fluid (the fluid that surrounds the foetus in the womb) for examination.

Amniocentesis is usually carried out after week 15 of pregnancy, and it can be carried out up until week 22. The sample of amniotic fluid is taken by passing a needle through your abdomen (tummy) and womb. The sample is drawn out through a syringe. 

As with CVS, the needle is guided into the correct position using an ultrasound scan. After the sample of amniotic fluid has been taken, it will be sent to a laboratory for testing.

Possible complications of amniocentesis can include:

  • infection
  • injury to you or your baby

As with CVS, there is also a 1 in a 100 chance of having a miscarriage after the procedure.

See the Health A-Z topic about Amniocentesis for more information.

Counselling

Being told that your baby may have Down’s syndrome can be difficult news to deal with. You will therefore be offered counselling so that you and your partner can express your feelings and ask questions about how this diagnosis may affect you both.

A counsellor will tell you which options are available to you so that you can make an informed decision about how to proceed with the pregnancy.

Diagnosis after birth

Once your baby is born, the initial diagnosis of Down’s syndrome is usually based on your baby’s physical appearance. Your doctor will assess your baby and look for the typical physical features that are associated with the condition, such as eyes that slant upwards or a flat back of the head.

If your doctor needs to definitively diagnose Down’s syndrome, they will arrange for a blood test known as a chromosomal karyotype. A sample of your baby’s blood will be taken and sent to a laboratory so that the chromosomes in the blood can be analysed. If the blood test finds that there is an extra chromosome 21, your baby will be diagnosed with Down’s syndrome. 

  • show glossary terms

Blood test
During a blood test, a sample of blood is taken from a vein using a needle, so it can be examined in a laboratory.
Chromosome
Chromosomes are the parts of a body cell that carry genes. A human cell usually has 23 pairs of chromosomes.
Gene
Genes contain information that you inherit from your parents, such as eye or hair colour. They are carried by chromosomes.

Last reviewed: 05/05/2010

Next review due: 05/05/2012

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Andrew Judd said on 06 November 2011

your text is again confused and misleading.

"All pregnant women, no matter what their age, should be given the choice to have screening for genetic conditions such as Down’s syndrome.

Antenatal screening is a way of assessing whether your unborn baby could develop, or has developed, an abnormality during your pregnancy.

Antenatal screening cannot diagnose conditions such as Down’s syndrome, but it can determine the likelihood of your baby developing the syndrome.

If the risk of Down’s syndrome (or any other condition) is shown to be high, further testing can be arranged to confirm whether or not your baby has the condition. The test that is used for Down’s syndrome is a combined blood test and an ultrasound scan, which is known as the ‘combined test’.

The 'combined test' is just a statistical method used in screening. Your text directly implies it is the diagnostic test used to investigate a postive screen test.

Please get these pages changed so people dont have to go thru what we went thru after being told our pregnancy was normal.

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Andrew Judd said on 06 November 2011

Your text on reliability of screening says:

"•a false-negative result, where a test gives an ‘all-clear’ by indicating that no problems have been found, but in reality the test has failed to detect a problem, such as the child having Down’s syndrome "

This text is horribly misleading. 'All clear' suggests *all* was clear. In reality all was not even tested to get the false negative result in screening.

By the way i dont live in the UK. I live in Finland. In Finland the tests seem to work in a similar way. Here even the paedeatrics department

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