What does Genetic Screening test for?

Genetic screening tests for three specific conditions

• Trisomy 21 (Down syndrome) - an extra copy of chromosome 21

• Trisomy 18 (Edward syndrome) - an extra copy of chromosome 18

• Open neural tube defects (spina bifida, anencephaly) - the spine or brain does not form properly, leaving parts of the spinal cord or brain exposed

What does Genetic Screening not test?

Genetic screening only tests for the three conditions listed above. It cannot tell you about heart defects, FASD, other chromosome abnormalities, or genetic disorders (such as cystic fibrosis, G6PD). Your detailed ultrasound might provide some more information about some of these (such as heart defects or physical malformations) but not all.

What are the reasons for having or not having genetic screening done?

There are many reasons that women choose whether or not to have genetic screening done.

Some women do not want genetic screening at all. They often feel that genetic screening and its results would not change how the proceed with the pregnancy or prepare for delivery.

Some women want genetic screening so that they can prepare for a baby with genetic abnormalities. This might include educating themselves on their baby’s condition or connecting with supports.

Some women want genetic screening because it may affect their decision to continue a pregnancy. This is a very personal choice and women should feel supported to make the decision that is right for them.

How is genetic screening done?

Genetic testing is typically done as two blood tests, one between 9 and 13 weeks, one between 15 and 20 weeks.

The ideal timing for these tests is 10-11 weeks 6 days, and 15-16 weeks 6 days.

These blood tests will measure certain proteins from the baby and placenta that are present in your blood. Using these levels, we can assess how likely your baby is to have one of the tested abnormalities.

How certain is genetic screening?

Genetic screening does not give a yes or no answer about abnormalities. Instead, it gives a risk of how likely this is. If your risk is calculated as higher than a cut-off value, then your screen is considered positive.

If your screen is positive, you may be offered additional testing to further check your baby for these abnormalities.

What happens if my genetic screening is positive?

Your doctor will discuss these results with you and which of the conditions was positive on screening. They will then discuss with you the next options for testing.

NIPT (Non-Invasive Perinatal Testing)

This is a second blood test specifically looks at baby’s DNA. Some of your baby’s DNA travels through the placenta and can be detected in your blood. By testing this, we can check for trisomy 21, trisomy 18, and also trisomy 13. This type of testing cannot test for open neural tube defects.

This testing will be offered if you screen positive for trisomy 21 or trisomy 18.

Some women choose to have this test done instead of genetic screening, as it can give a more certain answer about chromosome abnormalities. It is not covered by the provincial health care plan and costs about $500 to pay for it yourself.

Amniocentesis or Chorionic Villus Sampling

These are tests that take samples of your amniotic fluid or blood directly from the placenta (which includes baby’s blood). This is done to get a certain diagnosis of a genetic abnormality. It is more invasive and does come with certain risks - if you need this testing, you will be referred to a specialist obstetrician to discuss this testing further before it is done.