Advanced Prenatal Diagnostic Tests

Amniocentesis

Amniocentesis is a very accurate diagnostic test used to identify certain birth defects and genetic conditions in a fetus.

During the procedure, a small sample of the amniotic fluid surrounding the fetus in the uterus is drawn for testing.

You may choose to have an amniocentesis if your health-care provider says you are at risk of having a baby with conditions such as the following:

• Down syndrome
• Trisomy 18
• Cystic fibrosis (CF)
• Spina bifida
• And others

What to expect

Amniocentesis can be performed by an obstetrician (OB) after you are 15.5 weeks pregnant. The procedure takes about five minutes and is followed by a 30-minute resting period.  

Using ultrasound for guidance, the OB will carefully insert a thin needle through your belly and into your uterus to remove about one tablespoon of amniotic fluid from the area around the fetus. Removing this small amount of fluid is harmless to both you and the fetus.  

If you have an Rh-negative blood type, you will receive an immunoglobulin injection at the time of the procedure to prevent the development of antibodies that could harm the fetus.

Some patients say they feel no pain during the amniocentesis, while others say they feel some pressure and cramping when the needle enters the uterus. After the procedure, you may experience cramping, spotting or leakage of amniotic fluid. We recommend you avoid lifting or standing on your feet for long periods of time for several hours following the procedure.

The amniotic fluid that is collected contains fetal cells. The cells are grown (cultured) in a laboratory for 10 to 12 days and then tested for genetic conditions and birth defects.  

You will usually hear about your results from your health-care provider within a few weeks.

Risks and limitations

Serious risks associated with amniocentesis are rare and may include the following:

• A slight increase in the risk of miscarriage by 0.1 to 0.3%
• Within the first 12 hours after the procedure, a possibility of leaking amniotic fluid, spotting or cramping
• Rarely, the possibility of developing an infection
• A very small risk of an injury to the fetus

Call your primary care provider or visit the nearest emergency room if you experience a fever, bleeding or persistent cramping after an amniocentesis.

While amniocentesis is a valuable diagnostic tool, it does not provide a complete picture of all the potential genetic conditions or birth defects that could affect the fetus. It primarily focuses on detecting certain genetic conditions.

In rare instances, amniocentesis may need to be repeated because we were not able to collect enough amniotic fluid or there were technical issues in the laboratory. This does not necessarily indicate a problem with the pregnancy.

Learn more about amniocentesis from Prenatal Screening Ontario.

Chorionic villus sampling (CVS)

CVS is a highly accurate diagnostic test that can diagnose certain genetic conditions in a fetus.

CVS involves taking a small sample of cells from the developing placenta so they can be tested.  

You may consider having CVS if a genetic counsellor says you are at risk for having a baby with genetic conditions such as the following:

• Down syndrome
• Trisomy 18
• Cystic fibrosis (CF)
• Sickle cell disease
• And others 

What to expect

CVS is usually performed by an obstetrician (OB) between the 11th and 14th weeks of pregnancy. The procedure takes about 20 minutes and is followed by a 30-minute resting period.  

During the procedure, the OB will use ultrasound to check on the fetus and guide the CVS. There are two approaches that can be used to collect the tissue sample, either through the vagina or through the abdomen.

Using the vaginal approach, a thin tube will be carefully inserted through the vagina and cervix into the uterus to collect a small sample of tissue from the placenta.

Using the approach through the abdomen, the OB will insert a thin needle through your belly into the uterus to get a small sample of tissue from the placenta.  

If you have an Rh-negative blood type, you will also receive a Rho(D) immune globulin injection at the time of the procedure. This prevents the development of antibodies that can harm the fetus.

Some patients may experience light cramping or spotting after the procedure. We recommend you avoid lifting or standing on your feet for extended periods of time for the next several hours after the procedure.

The collected placental tissue contains the same genetic information as the fetus. This tissue is grown (cultured) in a laboratory and then tested for certain genetic conditions.  

Your health-care provider will call you within two weeks to explain your test results.

Risks and limitations

Serious risks associated with CVS are rare and may include the following:

• A one per cent increase in the risk of miscarriage
• Spotting or cramping in the first two days after the procedure
• Rarely, the possibility of developing an infection

Call your primary care provider or visit the nearest emergency room if you experience a fever, bleeding or persistent cramping after CVS.

While there have been some reports linking CVS to limb abnormalities in fetuses, the World Health Organization reports that there is no increased risk when CVS is performed by an experienced physician after 10 weeks of pregnancy.

While CVS is a valuable diagnostic tool, it does not provide a complete picture of all the potential genetic conditions or birth defects that could affect the fetus.

For example, unlike the amniocentesis test, a CVS cannot detect open spina bifida (a birth defect involving the spine). CVS primarily focuses on detecting certain genetic conditions.

In some cases, CVS may need to be repeated if not enough fluid was collected or if there was difficulty interpreting the results. This does not mean there is a problem with your pregnancy.

Learn more about CVS from Prenatal Screening Ontario.