What is chorionic villus sampling (CVS) and why is it performed?

Chorionic villus sampling, or CVS, is a test that uses a sample of the placenta to find genetic or chromosomal diseases and birth defects. Healthcare providers typically recommend a CVS if the fetus is at risk of having a birth defect, based on early screening tests (maternal blood test or fetal ultrasound), the mother's health or family medical history. The procedure usually takes place between weeks 10 and 12 of pregnancy.

Who is offered CVS?

CVS is most often recommended for pregnant women who:

• Are over the age of 35 and more at risk of having a child with Down syndrome or other genetic conditions
• Received abnormal results from a first trimester screening test, such as a triple screen blood test, a maternal blood test used to screen for genetic disorders such as Down syndrome or neural tube defects such as spina bifida or anencephaly
• Have had a child – or baby not carried to term – with a genetic birth defect
• Have – or whose husbands have – a family history of certain diseases or birth defects

How is CVS performed?

Healthcare providers perform the test using one of two methods, depending on the location of the placenta. Both techniques use ultrasound to pinpoint the placenta’s position, which determines whether the sampling needle is inserted through the cervix (transcervical) or the abdomen (transabdominal).

The physician removes a bit of tissue from the placenta (which contains the same genetic material as the fetus) using a syringe attached to a catheter (transcervical CVS) or a needle inserted through the woman’s abdomen (transabdominal). The sample is then examined in a laboratory.

CVS is typically an outpatient procedure and lasts about an hour. Providers may recommend an ultrasound two to four days after the procedure to make sure the pregnancy is proceeding normally.

What might the results show?

CVS can diagnose some genetic or chromosomal disorders, such as Down syndrome, earlier in the pregnancy than amniocentesis can. This allows couples to better prepare emotionally for the baby’s birth or consider terminating the pregnancy at a safer time. Full test results are usually available in one to two weeks.

An "abnormal" CVS means that the fetus may have at least one of more than 200 disorders. However,, a CVS cannot determine if there is a problem with the way the fetus is forming, so some disorders, such as spina bifida, can go undetected.

What side effects and risks are involved with the test?

Transcervical and transabdominal CVS are equally safe. For maximum safety, look for a facility experienced in both types of testing. Women typically experience cramping or light vaginal bleeding for a few hours after the procedure. In rare cases, uterine infections occur. Heavy bleeding, fever or contractions are not normal and may indicate an infection that requires a doctor’s attention.

In very few instances, CVS may cause miscarriage or membrane rupture. Studies show that the risk of miscarriage following CVS is similar to the miscarriage rate following amniocentesis – between 0.5% and 4.6%.

Other risks include:

Rh sensitization —This occurs when a mother does not have a type of protein called Rhesus (Rh) factor in her blood (Rh negative), but her fetus does have the protein (Rh positive). An Rh negative mother’s immune system will produce antibodies to attack the baby’s Rh positive blood cells. CVS is an invasive procedure, so women who are Rh negative may receive a Rh immunoglobulin shot to prevent Rh sensitization after the procedure.

Limb damage to the fetus—There is a risk of damaging the fetus's appendages (arm, leg or finger) when a CVS is done before week 10 of the pregnancy. CVS is now routinely offered after the 10th week; recent studies show the chances of CVS causing limb damage at this time is very unlikely.

What is the outcome of CVS?

CVS can provide early diagnosis of chromosomal abnormalities and certain genetic defects, which gives parents more time to prepare and consider their options. As with amniocentesis, most women who have the test receive reassuring results.

CVS test outcomes are very accurate (greater than 99%) in ruling out chromosomal disorders, although the results are inconclusive slightly more often than from amniocentesis tests. Therefore, some women also choose amniocentesis later in their pregnancy to answer questions not covered by CVS tests.

It’s important to note that no prenatal test can guarantee a healthy baby. Doctors use prenatal diagnostic tests to detect birth defects and sometimes treat these conditions before birth. Many couples are reassured by the results from amniocentesis and CVS. However, the tests can also raise difficult issues about the pregnancy and baby’s birth. Guidance from physicians, genetic counselors and spiritual advisors can be helpful in making decisions.