What is Chorionic villus sampling (CVS)?

Chorionic villus sampling is a prenatal test. The test involves taking a sample of chorionic villi that is removed from the placenta to be tested. The sample can be taken via the abdominal wall (transabdominal) or the cervix (transcervical).

During pregnancy, the placenta provides nutrients and oxygen to the growing baby and removes waste products from the baby's blood. The chorionic villi are pieces of placental tissue that share the baby's genetic makeup.

Chorionic villus sampling helps reveal whether a baby has a genetic condition or chromosomal condition such as Down's syndrome. The test can often be done as early as 10 weeks of pregnancy. Chorionic villus sampling often provides valuable information about a baby's health, but it's important to understand the risks and to be prepared for the results.

The chorionic villus sampling test is carried out by a specialist obstetrician.

Why is chorionic villus sampling done?

Chorionic villus sampling is usually offered when the test results might have a significant impact on the desire to continue the pregnancy or management of the pregnancy. Chorionic villus sampling helps reveal information about your baby's genetic makeup.

Chorionic villus sampling is often done between weeks 11 to 14 of pregnancy.

You might consider chorionic villus sampling if:

• You're 35 or older. Babies born to women over 35 have a higher risk of chromosomal conditions such as Down's syndrome.
   
• The result from a prenatal screening test came back positive. If the results of a screening test (for example, prenatal cell-free DNA screening or the first-trimester screen)— are positive or worrisome, you may choose to have chorionic villus sampling to confirm or rule out a diagnosis.
   
• A child was born with a chromosomal condition in a previous pregnancy. If a previous pregnancy was affected by a chromosomal condition such as Down's syndrome, another pregnancy might be at a slightly higher risk, too.
   
• Your partner has a genetic condition/is a known carrier or you have a family history of a specific genetic condition.

Chorionic villus sampling cannot detect certain birth defects, such as neural tube defects. If neural tube defects are a concern, genetic amniocentesis or ultrasound might be recommended instead.

You may be cautioned against transcervical chorionic villus sampling — which is done through the vagina — if you have:

• A vaginal or cervical infection, such as herpes.
• Vaginal spotting or bleeding in the previous two weeks.
• An inaccessible placenta, due to noncancerous growths or a tilted uterus in your cervix.
• Rarely, your doctor may advise against transabdominal chorionic villus sampling if your uterus is titled backwards and your placenta is located at the back of your uterus

How do you prepare for chorionic villus sampling?

It's likely you will need to have a full bladder in order to be able to have chorionic villus sampling. You will need to know how much fluid you would need to drink, as well as any other pre-testing preparation that might be necessary before the testing. This can be discussed with the medical specialist seeing you.

What do abnormal results mean?

The medical specialist will help you understand your chorionic villus sampling results. Occasionally, test results are unclear and amniocentesis (another prenatal diagnostic test) is needed to clarify the diagnosis. There's a rare chance of receiving a false-positive test from chorionic villus sampling.

If chorionic villus sampling reveals that your baby has a genetic condition that can't be treated, you might be faced with heartbreaking decisions — such as whether to continue the pregnancy. You will want to seek advice from family and friends.