Genetic testing in pregnancy

Most babies are born healthy but there is always a risk of that something can go wrong.

The risk will be greater for some couples than others depending on age and lifestyle issues and medical history.  

One of the most common genetic problems that a baby can have is a genetic condition called Down syndrome. While women are generally advised to have genetic testing, the decision to do so is yours. It’s a good idea to think about why you want to be tested and the consequences for you and your family if you learn that your baby does have a genetic disorder.

You are more at risk of having a baby with a genetic disorder if:

• you are 37 years of age and over
• you or your partner have previously had a baby with a genetic problem.

Screening tests are best done in the first 16 weeks of pregnancy and cannot be done after 19 weeks.

There are two kinds of tests that can be done in pregnancy.

1. Screening tests can tell you if you are at risk of having a baby with birth defects. These tests will not give you definite information about your fetus.
2. Diagnostic tests can tell you if the fetus has a defect.

Women can choose whether or not to have tests to find out their risk of having a baby with a birth defect. Some hospitals have genetic counselling services who can discuss with you the implications of having testing done and what it might mean to you.

Some of these tests need to be done in early pregnancy, if you are a public patient you may need to organise testing them with your doctor (GP). 

Screening tests

First trimester combined screening test

This test combines the results of a blood test taken at around 10-12 weeks and an ultrasound at 11-13 weeks. The test will show the risk or your chance of having a baby with Down syndrome or Trisomy 18. It will not tell you if your baby has Down syndrome.

If you are at increased risk you will be offered a diagnostic test, either a CVS (Chronic Villus Sampling) or amniocentesis.

Maternal serum screening

This is a blood test collected between 15-20 weeks of pregnancy. The test shows your risk of having a baby with Down syndrome, Trisomy 18 or neural tube defects such as spina bifida. If the test shows you are at an increased risk you will be offered amniocentesis and ultrasound.

Non-invasive prenatal test (NIPT)

This blood test is done after week 10 of pregnancy. It screens for Down Syndrome and certain other chromosomal irregularities in a baby. In Australia it is only available in some specialist centres.

Diagnostic tests

A diagnostic test is testing the fetus’s genetic material and can therefore tell whether the fetus actually has a genetic disorder.

Chorionic Villus Sampling (CVS) (11 to 12 weeks)

In this test a small sample is taken from the placenta. The sample from the placenta can be tested for Down syndrome or in some cases other genetic conditions such as cystic fibrosis. One woman in one hundred (1%) will have a miscarriage as a result of this test.

Amniocentesis (15 to 18 weeks)

A sample of the amniotic fluid that surrounds the baby is collected and can be used to diagnose Down syndrome or some other genetic conditions. Amniocentesis has a one in two hundred risk of causing a miscarriage.

Ultrasound scans (18 to 20 weeks)

This second trimester scan is used to identify physical and structural abnormalities including spina bifida, heart and limb defects.

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