Chorionic villus sampling, or CVS, is one type of invasive prenatal diagnostic testing that is available in Ontario.

CVS can determine if a pregnancy has a chromosome difference like trisomy 21, trisomy 18 or trisomy 13. It can also give information about whether there are sections of other chromosomes that are extra or missing. These other chromosome differences are not associated with an increase in maternal age (or age of egg donor), and can run in families or occur spontaneously in the sperm or the egg that conceived the pregnancy. Additional genetic conditions can be tested if it is indicated for that pregnancy, and requested by the pregnant individual.  

When CVS is done

A CVS is typically offered in the first trimester of pregnancy and is typically performed between approximately 11 and 14 weeks of pregnancy.

How it works

CVS procedures are performed at specialized centres in Ontario by specially trained physicians. An ultrasound is performed to show the physician the location of the pregnancy and of the placenta.

There are two types of CVS procedures that can be done, depending on the position of the placenta and the expertise of the obstetrician:

• Transcervical. This is the more common of the two procedures, where a special tool is passed through the vagina, and a small piece of tissue is taken from the placenta.  

• Transabdominal. In this procedure, a needle is inserted through the abdomen and uterus into the placenta to obtain a small piece of tissue.

Transcervical chorionic villus sampling (CVS)

Illustration adapted from Genetic Counseling Aids, 7th Edition, Copyright 2020, permission for use granted by Greenwood Genetic Center

Results

The tissue from the placenta obtained through CVS will then be sent to the testing lab for analysis.  CVS results will be sent directly to the health care provider who ordered the test; results will typically be available within 2-3 weeks but can vary depending on the type of testing that is ordered. Individuals should confirm with their health care provider when to expect results.

Benefits of CVS

• can give a “yes” or “no” answer about chromosome changes
• provides results earlier than amniocentesis because CVS is performed in the first trimester of pregnancy

Limitations of CVS

• has a procedure-related miscarriage risk of <1%. It is important to understand that all pregnancies have some risk of miscarriage in the first trimester. CVS increases this risk by <1%.
• can generally only provide information about chromosome differences in the pregnancy (and other specific genetic conditions in specific circumstances). CVS cannot identify every possible genetic condition. CVS will also not tell you about whether there are any structural abnormalities, that may be detected by ultrasound instead (e.g. heart conditions).

Mosaicism

The baby and the placenta began from the same initial fertilized cell and will usually have the same chromosome arrangement. In about 1-2% of all pregnancies, however, the placenta can have a different chromosome makeup from the baby. This is called confined placental mosaicism (CPM).

If a CVS result shows that a chromosome is extra or missing, it is possible that the extra or missing chromosome is only present in the placenta and not the baby. If CPM is suspected, your health care provider may recommend an amniocentesis procedure be performed as a follow-up test. Amniocentesis looks at the cells derived directly from the baby, not placental cells, and can clear up any concern about CPM.

How to get more info

If you have questions about invasive diagnostic testing, contact your Regional Genetics Centre or Prenatal Screening Ontario for more information.