Screening Tests for Down syndrome

Down syndrome

Screening tests for Down syndrome

What can the tests tell?

How reliable are the tests?

What happens if your baby is found to have Down syndrome?

Where can you get more information?

 

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. It is important before you have the tests to think about the possible results and what you might do about them. You can discuss this with your doctor or midwife.

If your chance of having a baby with this problem is increased because of your age or family history, you may wish to consider amniocentesis or chorionic villus sampling rather than screening tests.

Down syndrome

Down syndrome is a disorder the baby is conceived with. It arises when the baby has an extra chromosome (chromosome 21). It is not known why this happens.

Affected children have intellectual disability and may have physical problems. The degree of problems varies, with some people needing a lot of assistance with daily activities and some living almost independently. Some will need medical treatment for physical problems.

Screening tests for Down syndrome

Maternal screening cannot give you a definite answer to the question 'does my baby have Down syndrome?' but it will show whether your unborn baby is more likely than usual (at increased risk) to have Down syndrome.

About 19 out of 20 women (95%) who have the screening tests will get a report saying their baby is not at increased risk of having Down syndrome. About 1 in 20 (5%) will get a report that there is an increased risk.

There are currently two choices for Down syndrome screening.

First trimester screening

First trimester screening involves

• A blood test done between 10 weeks and 13 weeks and 6 days.
• An ultra sound (called Nuchal Translucency – looking at the back of your baby’s neck) done between 11 weeks and 13 weeks and 6 days. To have this ultrasound test you will usually need a referral from your family doctor to a private radiology practice.

The results from each of these tests are combined to work out your risk of carrying an affected baby.

Second trimester screening

This screening test can be done between 14 weeks and 20 weeks and 6 days. It is a blood test from which your risk can be calculated. This blood sample can also be tested to see if your baby may have an increased chance of having a neural tube defect.

What can the tests tell?

A report which says ‘not at increased risk’ means there is only a very small chance that your baby has Down syndrome. This does not guarantee a perfect baby but almost all pregnancies identified as not at increased risk will not have Down syndrome. (Note: your baby may have a problem which is not screened for.)

If there is an increased risk after the first trimester screening, 95% of these babies will not have Down syndrome. After the second trimester screening, 98% of babies who are at ‘increased risk’ will not have Down syndrome. Only further testing will show which babies do have the problem.

Depending on the results you may need to decide whether or not to have further testing. These further tests are:

• Chorionic villus sampling (CVS)
• Amniocentesis

CVS and amniocentesis have some risks - your doctor or midwife will be able to discuss them with you. These tests are very accurate.

You do not have to have this further testing - it is your choice. Discuss this with your doctor or a trained counsellor.

How reliable are the tests?

• If done between 10 and 13 weeks pregnant, the blood test and ultra sound scan together will detect around 90% of babies affected with Down syndrome.
• If the blood test is done between 15 and 20 weeks it will identify about 75% of babies with Down syndrome.

What happens if your baby is found to have Down syndrome?

If your baby is found to have Down syndrome you will be given information about how this might affect your baby and the rest of your pregnancy, as well as the support services that are available.

You will then have to decide whether to continue your pregnancy or end it early.

Where can you get more information?

• Talk to the doctor who is looking after you during your pregnancy.
• Contact the Down Syndrome Association in your state or territory.