Down's syndrome - Screening for and diagnosing Down's syndrome

Screening during pregnancy can determine the likelihood of a baby being born with Down's syndrome, but in many cases the condition is only diagnosed after birth.

Screening during pregnancy

All pregnant women should be offered screening for genetic conditions such as Down's syndrome. Antenatal screening assesses the likelihood of your baby developing abnormalities.

Screening cannot definitively diagnose Down's syndrome, but it can help with the decision to have further tests. However, no test is 100% accurate and it's possible for potential problems to remain undetected during screening.

Screening for Down's syndrome should ideally be carried out before the end of your first trimester (14 weeks and one day). Most women are screened between 11 and 14 weeks. However, if necessary, it's possible to have screening up to 20 weeks into your pregnancy (this will involve a different test – known as the "quadruple blood test").

Combined test

The main screening test used for Down's syndrome is known as the "combined test". It includes:

• a blood test to check the levels of certain proteins and hormones, as abnormal levels of these substances could mean there is an increased risk of your baby having Down's syndrome
• an ultrasound scan (known as a nuchal translucency scan) to measure the size of the pocket of fluid behind your baby's neck, as babies with Down's syndrome usually have more fluid in their neck than normal

The results of these two tests will be used, along with your age, to help determine the chances of your child having Down's syndrome. If this risk is found to be high (at least a one in 150 chance), you will be offered further tests to confirm whether your baby has the condition.

Quadruple blood test

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 and 20 weeks.

This test is called the quadruple test because it measures four hormones 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.

Diagnosis during pregnancy

There are two main tests used to diagnose a baby with Down's syndrome while it's still in the womb. These are:

• chorionic villus sampling (CVS) – a small sample of the placenta (an organ attached to the lining of your womb during pregnancy) is removed with a needle or tube for examination; this is usually done from week 11 of pregnancy
• amniocentesis – a small sample of amniotic fluid (the fluid that surrounds the foetus in the womb) is removed with a needle for examination; this is usually carried out from week 15 of pregnancy

Make sure you discuss the benefits and risks of having either of these procedures with your doctor or midwife beforehand though, as both tests are thought to have around a one in 100 chance of causing a miscarriage.

Getting the results

The results of CVS or amniocentesis are normally available within a few days.

If the test shows that your baby is unlikely to have Down's syndrome, your pregnancy should continue as normal.

If you are told that it's likely your baby does have Down's syndrome, you should be offered genetic counselling to allow you and your partner to discuss the impact of the diagnosis on you, and talk about the available options.

You may also be offered an appointment to meet a doctor or other health professional who works with children with Down’s syndrome, who can also tell you more about the condition and answer any questions you may have.

This will help you make an informed choice about how to proceed with the pregnancy.

Diagnosis after birth

Once your baby is born, an initial diagnosis of Down's syndrome is usually based on your baby's appearance, as there are often clear physical characteristics of Down's syndrome.

If your doctor needs to confirm the diagnosis, a sample of your child's blood can be taken and analysed to look for the extra copy of chromosome 21 that causes Down's syndrome.