Prenatal Genetics


Prenatal genetics includes genetic counselling and testing for a variety of conditions which can be found for a developing baby during a pregnancy. This can include inherited and non-inherited conditions. Some conditions can occur in a baby regardless of medical history while others can be passed down in a family.

Most babies are born healthy. Some babies have an increased chance of having a genetic condition. Some of these concerns are identified before a woman becomes pregnant, such as a family history of a genetic condition. Others may be identified during the pregnancy through a test in the pregnancy, such as an ultrasound.

Who could consider a genetics consultation?
How to make an appointment in Genetics
What to expect during a genetic counselling appointment
Information about prenatal screening and testing in Ontario


Who could consider a genetics consultation?

  • a woman with positive prenatal screening result (FTS, IPS, MSS)
  • a woman with abnormal ultrasound findings in a pregnancy
  • an individual with personal or family history of a known, or suspected genetic condition
  • an individual with personal or family history of a chromosome rearrangement
  • an individual with previous child with a birth defect or chromosome problem
  • an individual with questions about the risk of a genetic condition for their children
  1. NOTE:

    As of March 1, 2010, Clinical Genetics is no longer accepting referrals for women whose reason for referral is being 35 years of age, or more, at the time of delivery.
    For more information about the testing choices available to you, feel free to view our prenatal screening and testing video». If you have any questions or wish to have one of these tests, please discuss this further with your doctor or midwife.

How to make an appointment in Genetics

An individual requires a referral from their health care provider to be seen in Clinical Genetics. For more information, see How to make an appointment page»

What to expect during a genetic counselling appointment:

An individual will meet with a genetic counsellor or geneticist, or sometimes both. During the appointment a family history will be taken. The genetic counsellor or geneticist will discuss the reason for referral and explain the results of any testing done prior to the appointment. The testing options available will be discussed which might include: ultrasound, prenatal screening, prenatal testing, genetic testing.

The genetics team is dedicated to working with women to help them understand complex information about some tests available in a pregnancy to look at the health of the developing baby. The choice to have testing is always up to the woman.

Information about prenatal screening and testing in Ontario

Prenatal Screening:

Family history: Individuals with a genetic condition, or a family history of a genetic condition, might have an increased chance of having a child with the same condition. Genetic counselling is available to review these chances and testing options.

Population Screening: Some genetic conditions, such as thalassemia and sickle cell anemia, are more common in certain parts of the world. People whose ancestors are from these areas may have an increased chance of having a child with one of these conditions. Genetic counselling is available to review testing options if a couple both are found to be carriers. See Information about hemoglobinopathies pamphlet [pdf]

Ultrasound: Ultrasound can be used to look at the developing baby for normal growth and development. Ultrasound can be done at different times in a pregnancy. It might find a baby has an increased chance of a health problem. Many times more testing would be offered. Genetic counseling is available to review these chances and testing options.

Age: All women have a chance of having a child with a chromosome difference. This chance increases slightly each year as a woman gets older. The most common condition is Down syndrome (Trisomy 21). Currently, the Society of Obstetrics and Gynecology in Canada recommends that " maternal age screening is a poor minimum standard for prenatal screening for aneuploidy (chromosome difference ) and should be removed as an indication for invasive testing (amniocentesis or CVS)". You may wish to discuss this further with your healthcare provider.

Multiple Marker Screening: Multiple marker screening provides information about the chance of having a baby with Down syndrome, trisomy 18 and an open neural tube defect. Multiple marker screening is available to all pregnant women in Ontario. There is no risk to the baby by having multiple marker screening.

It is a woman’s choice whether or not she wants to have prenatal screening. Different options for multiple marker screening are available. Below is a chart showing the different screening tests and when they are done. More information about each test is available at provincial patient prenatal pamphlet [pdf]» or by watching this video»

*the ultrasound that is part of FTS and IPS usually is done between 11-14 weeks. The ultrasound looks at the size of the baby, the fetal heart beat, and measurement of the nuchal translucency (NT).

For a list of ultrasound units that perform NT ultrasound that can be accepted by the FTS/IPS program see IPS/NT ultrasound units [pdf]»

NoteFactors that can affect a prenatal screening result: Update for health care providers – Prenatal Screening in Ontario
This update contains information about three aspects of prenatal screening – maternal weight, maternal smoking and open neural tube defects results.»

Prenatal Diagnostic Testing:

There are two common prenatal diagnostic tests available, called chorionic villus sampling (CVS) and amniocentesis. Below is a description of each test. You can also get more information about them by watching this video»

Chorionic Villus Sampling (CVS):

Chorionic villus sampling (CVS) is a test that looks at the chromosomes of a baby during pregnancy. It is usually done between 11 and 14 weeks of pregnancy. During the test, an obstetrician either puts a small tube through the vagina into the uterus, or puts a thin needle into the woman’s belly. A piece of the placenta (chorionic villi) is taken. Testing is done on the sample. CVS can tell if a baby has Down syndrome or trisomy 18.

CVS is considered to be a safe test for both a woman andher baby. Any woman has a 4% chance of having a miscarriage at this time of pregnancy. If a woman has CVS, this chance is increased by an extra 1 out of 100 or 1%.

CVS cannot look for open neural tube defects. However, a blood test can be done at around 16 weeks of pregnancy to measure alpha-fetoprotein (AFP). This blood test can tell if there is an increased chance of a baby having an open neural tube defect. Also, an ultrasound of the developing baby done between 18 and 20 weeks of pregnancy can find many neural tube defects.

This test is not available in the Peel region. It is offered through Mt. Sinai Hospital in Toronto.

Amniocentesis:

Amniocentesis is a test that looks at the chromosomes of a baby during pregnancy. It is usually done between 16 and 22 weeks of pregnancy. During the test, an obstetrician passes a thin needle into the woman’s belly and into the fluid around the baby. A small amount of fluid from around the baby is taken. Testing is then done on the fluid. Amniocentesis can tell if a baby has Down syndrome or trisomy 18.

Amniocentesis is considered to be a safe test for both a woman and her baby. Any woman has a 2-3% chance of having a miscarriage at this time of pregnancy. If a woman has amniocentesis, this chance is increased by about an extra 1 out of 200 or 0.5%.

Amniocentesis can test for open neural tube defects. This is done by measuring a protein called alpha-fetoprotein (AFP).