Chorionic villus sampling (CVS) is a prenatal test used to detect birth defects, genetic diseases, and other problems during pregnancy. During the test, a small sample of cells (called chorionic villi) is taken from the placenta where it attaches to the wall of the uterus.
Chorionic villi are tiny parts of the placenta that are formed from the fertilized egg, so they have the same genes as the baby.
You may be offered CVS if you have certain risk factors for having a baby with a birth defect or genetic disease, so that problems can be found early in pregnancy. Many blood screening tests for birth defects are less reliable if you're having twins. CVS can give you some clear answers. However, the risks of CVS are higher in women with twins.
What Diseases or Disorders Can CVS Identify?
CVS can help identify such chromosomal problems as Down syndrome or other genetic diseases such as cystic fibrosis, Tay-Sachs disease, and sickle cell anemia. CVS is considered to be 98% accurate in the diagnosis of chromosomal defects. The procedure also identifies the sex of the fetus, so it can identify disorders that are linked to one sex (such as certain types of muscular dystrophy that occur most often in males).
CVS does not detect open neural tube defects like spina bifida.
What Are the Benefits of CVS?
CVS can be done early in pregnancy (earlier than amniocentesis), and results are usually obtained within 10 days. Getting this kind of information early allows a woman to make choices in the beginning stage of their pregnancy. If a woman chooses to terminate the pregnancy after receiving abnormal test results, the termination will be safer than if they wait until later for amniocentesis results.
What Are the Risks of CVS?
CVS may carry a slightly higher risk of miscarriage than amniocentesis, because the procedure is done in early pregnancy. Infection may also occur. Rare cases of defects in baby's fingers or toes have been reported, especially when CVS was done before 9 weeks. This risk seems unrelated to the doctor’s experience or how the test was done. Due to this potential risk, 10 weeks is generally the earliest recommended time to perform this test.
CVS is a little trickier with twins than it is with single babies. Talk to your doctor about the risks. Make sure the doctor has a lot of experience doing CVS with twins.
Who Should Be Tested With CVS?
The American College of Obstetrics and Gynecology recommends offering CVS when there is an increased risk for a genetic disorder in the baby. This may include:
- Pregnant women who will be age 35 or older on their due date (the risk of having a baby with a chromosomal problem such as Down syndrome increases with the age of the woman)
- Couples who already have had a child with a birth defect or have a family history of certain birth defects
- Couples with a parent known to carry a chromosomal abnormality or genetic disease
- Pregnant women with other abnormal genetic test results
Your health care provider can advise you on whether this test is right for you. In the end, only you and your partner can decide whether you should have this test performed.
How Is the CVS Test Performed?
Before undergoing a CVS prenatal test, appropriate genetic counseling, including a detailed discussion regarding the risks and benefits of the procedure, are recommended.
At the time of initial consultation and counseling, an ultrasound exam will be performed to confirm gestational age (the development stage of the embryo) and the location of the placenta. This is done so that CVS can be performed at the appropriate gestational age (which is usually 10 to 12 weeks from the woman's last menstrual period).
There are two ways to collect chorionic villi from the placenta: through the vagina or through the abdomen.
To collect cells through the vagina, a speculum is inserted (in the same way as a Pap test). Then a very thin, plastic tube is inserted up the vagina and into the cervix. Using ultrasound images, the tube is guided up to the placenta, where a small sample is removed.
To collect cells through the abdomen, a slender needle is inserted through the woman's abdomen to the placenta, much like in amniocentesis.
The sample of chorionic villi is then sent to a lab, where the cells are grown in a special fluid and tested a few days later. Culture results will be available within 2 weeks. Your doctor will notify you of the results.
Will the CVS Test Be Painful?
It could be, but it's over relatively quickly. The test takes no longer than a half hour from start to finish. The extraction itself runs only a few minutes.
What Happens After the CVS Test?
You'll need to take it easy immediately after a CVS test, so arrange for someone to drive you home. For the rest of the day, you'll need to rest. Generally, women are advised to abstain from strenuous physical activity, sex, and exercise for 3 days following the procedure. You may have some cramping and bleeding, which is normal, but do tell your doctor or midwife. If you notice fluid leaking from your vagina, call your health care provider immediately.
What to Know About Test Results
You should get test results in about 1 week. CVS is very accurate. If there is a problem, you can meet with a genetic counselor and talk about your options.