Chorionic villus sampling (CVS) is a diagnostic test that can be done during pregnancy to detect if an unborn baby has certain genetic conditions, such as Downs syndrome or cystic fibrosis. The procedure involves collecting a small sample of cells from the placenta which are then genetically tested. Usually this test will be only offered if you have had a ‘high risk’ result from a prenatal screening, or have a family history of a genetic condition, and is done between 11 and 14 weeks.

The placenta 101:

The placenta is the organ that supports a baby’s life inside the womb. It has a maternal side (from the mother) and a fetal side (from the baby) which brings the blood from the mother and baby close together. Blood from the mother and baby do not mix but because they are so near, oxygen and nutrients can pass to the baby and waste can pass to the mother to be eliminated.

The fetal blood vessels that come from the umbilical cord, attach into the placenta and form little structures that look like trees. These tiny fetal blood vessels are called chorionic villi and are made up of chorionic villi cells. Chorionic villi cells have the same DNA as the baby and are the cells selected for CVS sampling.

How are the cells collected?

Chorionic villi cells may be collected either via transabdominal sampling or transcervical sampling.

Transabdominal CVS is when a needle is inserted through the abdomen to reach the placenta. Sonography (ultrasound) is used during this procedure to guide the needle so that the correct cells are located, and the needle does not go near the baby or amniotic sac. You should be offered anaesthesia for this procedure.

Transcervical CVS uses a little suction tube which is inserted into the vagina and through the cervix to reach the placenta. This procedure also uses sonography for guidance and usually doesn’t require anaesthetic. The sampling is described as uncomfortable, similar to a cervical screening test (previously called pap smear) but not painful.

CVS is an outpatient/day procedure and the appointment may take about one hour, with sampling taking only about 5-10 minutes. Generally, it is advised that people who are having CVS, should rest for about 20 minutes after the procedure and have a support person to take them home. Some cramping or bleeding after the procedure is not unusual. Your doctor will discuss with you what to expect and what to look out for after having chorionic villus sampling. Results should be available within about two weeks. 

What are the benefits?

Chorionic villus sampling is more accurate than results from the first and second trimester prenatal screenings. Generally, if a prenatal screening result has returned a high-risk result, CVS can give a nearly definitive answer (99%) whether a baby does or does not have an abnormality. If the CVS result shows normal genes, the pregnancy can be continued without extra care. If the test diagnoses a genetic condition and the parents wish to continue with the pregnancy, future care can be planned around the individual needs of the family. For people who are unsure about continuing the pregnancy, termination is an option that may be considered.

CVS is one of two tests that can be done to determine genetic conditions. The second one, called amniocentesis, cannot be done until after 15 weeks of pregnancy, so CVS can give results earlier. This is important for couples who are likely to consider termination if an abnormality is found.

What are the risks?

• CVS is considered diagnostic, however the accuracy rate it is 99%. This means that there is a very small chance (1 in 100) that the result may be inaccurate. This may be important to consider if do not want to continue a pregnancy with a genetic abnormality.
• There is a risk of miscarriage related to CVS, at 1%. This means that in every 100 procedures, one pregnancy ends in miscarriage. Whether this is a high risk or low risk, should be weighed by you.
• About 1% of samples are inconclusive and do not give a result. If this happens, you may be offered chorionic villus sampling again, or you may wait until 15 weeks to have an amniocentesis instead.
• There is a chance that fetal blood may enter the mother’s bloodstream. This is not generally an issue for people with positive blood groups. However, it can cause the development of antibodies in women who are Rh negative. For women who are Rh negative, a dose of anti-D immunoglobin (anti-D injection) is recommended to prevent this from happening.
• Rarely, some babies are born with defects to the hands/feet or arms/legs. This usually occurs when the sample is done too early in pregnancy, and why CVS should not be done before 10 weeks.
• Other risks and side effects of CVS include, rupture of the amniotic sac (‘breaking the waters’), bleeding, infection, abdominal discomfort/cramping and nausea.

Summary

CVS is considered as a safe, timely and optional test to get very accurate genetic information about your baby. It is usually only preformed if you are in a high-risk group (eg. due to family history) or if earlier prenatal screenings have shown a high-risk result. Some couples will not want to have a CVS test, and that is okay. For other couples, a positive result from the CVS test can be used to prepare for having a baby with additional needs due to a genetic condition or may be used to make a decision to terminate the pregnancy.