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 are offered screening for Down's syndrome.

Screening cannot definitively diagnose Down's syndrome, but it can help determine the chances of your baby having Down's syndrome.

The screening tests are:

• a blood test plus nuchal translucency ultrasound scan (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

or

• 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

Read more about screening for Down’s syndrome in pregnancy, including when it’s carried out and the different tests used.

If the screening test results show that you have a higher risk of having a baby with Down's, you will be offered diagnostic tests to find out for certain if your baby has the condition.

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.