Screening for Down's syndrome

During your pregnancy you'll be offered a test to detect if your baby might have Down’s syndrome. All pregnant women, no matter what age, can be tested.

A blood test and nuchal translucency (NT) scan can be used to screen for this condition.

What's Down’s syndrome?

Down’s syndrome is a condition where a person develops an extra copy of chromosome 21. This usually happens by chance and isn't caused by anything parents do before or during pregnancy. It's sometimes inherited but this is very rare. Down's syndrome is sometimes known as Trisomy 21.

Older mothers are more likely to have a baby with Down’s syndrome, but it can occur in women of any age. Down’s syndrome occurs once in every:

• 1,500 births to women aged twenty or younger
• 900 births to women aged around thirty
• 100 births to women aged around forty

More about Down's syndrome

Testing for Down's syndrome

There are 2 stages to testing for Down’s syndrome:

• screening tests to find out if your baby has a high or low chance of having Down's syndrome (offered up to 20 weeks of pregnancy)
• diagnostic tests to confirm whether your baby has the condition (offered if your baby has a high chance result)

Your midwife will discuss the tests with you. It's a personal decision and you should take time to think about it very carefully. Talk with the health professional taking care of you, as well as your partner or people close to you. The final decision should be yours.

Screening tests

The type of screening tests you'll be offered will depend on the stage of your pregnancy:

• less than 14 weeks pregnant - a blood test combined with a special ultrasound called a nuchal translucency (NT) scan (known as the combined test)
• between 14 and 20 weeks - a blood test only

Sometimes your early scans will show you're at a different stage of pregnancy than you'd thought. If this happens, the tests you'll be offered will change.

Pregnancy screening choices in Scotland will be changing in Summer 2020. Check for the most up-to-date information.

Blood tests

The blood test measures substances that have passed between you and your baby.

A sample of your blood is usually taken between 11 and 20 weeks.

Several factors can affect the screening result, including:

• if you smoke
• if you have an assisted pregnancy (especially if it involves a donor egg or frozen embryo).

It's important to give the staff this information in order for you to be given the most accurate screening result possible.

A computer programme uses the results of your blood test, along with your age, weight, stage of pregnancy and any other relevant factors, to work out the chance of your baby having Down’s syndrome.

More about blood tests

Nuchal translucency scan

A nuchal translucency (NT) scan is an ultrasound scan carried out between 11 and 14 weeks, usually as part of your early pregnancy scan. If you're over 14 weeks at your early pregnancy scan, an NT measurement can't be taken and you'll only be offered a blood test on it's own.

During the scan:

• the amount of fluid lying under the skin at the back of your baby’s neck is measured
• a computer programme will use this measurement, and your blood test result, to work out the chance of your baby having Down’s syndrome

If you're having twins or triplets, an NT scan can be used on its own to work out the chance of each baby having the condition.

What happens if the scan can't be performed?

Sometimes the sonographer's unable to carry out the NT scan because your baby's in a position where the amount of fluid lying under the skin at the back of the neck can't be measured.

If this isn't possible:

• you'll only be offered the blood test on it's own between 14 and 20 weeks
• your early scan can still be performed, but an NT measurement won't be taken - the scan will continue as normal to allow the sonographer to date the pregnancy and say when the baby's due

Test results

If your baby has a low chance result for Down's syndrome, you won't be offered a diagnostic test. Most screening tests (more than 95%) return a low chance result.

It’s important to understand that:

• some babies with Down’s syndrome aren’t detected by screening tests, so a low chance result doesn't mean there's no chance your baby has the condition, just that it's unlikely
• about 1 in 10 babies with Down’s syndrome won't be detected by screening

High chance result

If the screening test shows a high chance result, you'll be offered a diagnostic test to confirm if your baby has Down’s syndrome.

Of the women screened, about 1 in 50 are offered a diagnostic test.

Diagnostic tests for Down’s syndrome

The second stage of testing for Down’s syndrome involves either:

• chorionic villus sampling (CVS)
• amniocentesis

These tests will show if your baby has Down's syndrome.

Having diagnostic tests can increase your chances of having a miscarriage. This is why they aren't offered to all women.

More about diagnostic tests for Down's syndrome