If you’re at higher risk of having a baby with a chromosomal disorder or a birth defect, your doctor may recommend a common genetic test called a chorionic villus sampling, or CVS.
CVS is a first trimester prenatal diagnostic test that involves taking a small tissue sample from the finger-like projections of the placenta, called the chorionic villi.[1]
This sample contains the same genetic information as your baby, so it can be used to check for chromosomal conditions like Down syndrome and abnormalities such as cystic fibrosis and muscular dystrophy with a high degree of accuracy. It can also determine the sex of your baby, so speak up if you don't want to know.
Test results can help you, your partner and your provider plan for the future, including choosing whether or not to move forward with the pregnancy, preparing yourself emotionally, and determining what kind of care or treatment your baby might need after she’s born.
Still, CVS testing isn’t for everyone — and it does come with some small risks. Here’s what you can expect if you’re considering the procedure, plus what you can learn from the results.
Who chorionic villus sampling is for
CVS testing is available to any pregnant woman — typically between weeks 10 and 13 of pregnancy. It’s not necessary for every mom-to-be, but your doctor may recommend CVS testing if you:
- Are over 35, and therefore at increased risk of carrying a baby with a chromosomal disorder
- Or your partner have a family history or are a known carrier of a specific genetic disease for which CVS tests
- Had a previous ultrasound that showed concerning findings
- Received uncertain results from a nuchal translucency screening, first-trimester blood tests or noninvasive prenatal testing (NIPT)
Keep in mind that even if your doctor recommends it, the decision to pursue CVS testing is entirely optional. If you’re having trouble deciding what’s right for you, your provider or a genetic counselor can help you weigh the risks.
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The difference between amniocentesis and CVS
Both CVS and amniocentesis are diagnostic tests aimed at detecting chromosomal disorders. Both have their benefits and drawbacks, so it’s important to talk with your provider about your options.
The main advantage of CVS is that it’s performed earlier than amniocentesis, giving you an opportunity to learn about a potential chromosomal disorder earlier. CVS testing is typically done between weeks 10 and 13 of pregnancy, while amniocentesis is usually done between weeks 15 or 16 and 20.[2]
That said, CVS does have its drawbacks. Amniocentesis can detect certain things that CVS can’t, including neural tube defects like spina bifida, birth defects, and Rh incompatibility. Possible risks, like miscarriage, are slightly higher for CVS, and the procedure is slightly more painful.
How chorionic villus sampling is done
CVS is most often performed by a maternal-fetal medicine specialist in an ultrasound suite. Depending on the location of the placenta, a sample of placental cells will be taken either through the vagina and cervix (transcervical CVS) or by inserting a thin needle into the abdomen (transabdominal CVS).[3]
Neither method is entirely pain-free. The discomfort can range from very mild to moderate, and some evidence suggests the transcervical procedure is less uncomfortable than the transabdominal one. Some women experience cramping (similar to menstrual cramps) when the sample is taken.
Both procedures take about 30 minutes from start to finish, though the actual withdrawal of cells takes no more than a minute or two.
- In the transcervical procedure, you'll lie on your back while a long, thin tube is inserted through your vagina into your uterus. Guided by ultrasound imaging, the doctor positions the tube between the uterine lining and the chorion, the fetal membrane that forms on the baby’s side of the placenta. A sample of the chorionic villi is then suctioned through a tube for diagnostic study.
- In the transabdominal procedure, you'll also lie tummy-up. Ultrasound is used to determine the location of the placenta and to see the uterine walls. Then, still with ultrasound guidance, a needle is inserted through your abdomen and the uterine wall to the edge of the placenta, and the sample is drawn up through a needle.
If Rh factor testing confirms you are Rh negative, you should get an injection of RhoGAM following CVS (since this is one of those procedures that might result in mixing the maternal and fetal blood).
Test results are usually available in two or more weeks.
What the results of a CVS test mean and how accurate they are
The chorionic villi sample can give a clear picture of the genetic makeup of the developing fetus, and detect conditions including Down syndrome (trisomy 21), trisomy 13, trisomy 18, triploidy, Tay-Sachs disease, sickle cell anemia and cystic fibrosis. CVS cannot, however, test for neural tube and other anatomical defects.
Is the test 100 percent accurate? No, but it’s close. CVS is able to accurately detect chromosomal abnormalities 98 percent of the time. If the results of your test show that your baby has a birth defect or chromosomal disorder, you and your doctor can talk about your options, including treatments like surgeries or medications that your baby might be able to receive before she’s even born.
Risks of CVS testing
CVS is generally considered to be safe and reliable. The procedure does carry a very small risk of miscarriage — about 0.2 percent compared to 0.1 to 0.3 percent for amniocentesis. In rare cases, CVS can cause an infection.
Thinking about these risks can be worrying, and there’s no way to get rid of them completely. But choosing a testing center with a good safety record and waiting until right after your 10th week can reduce the chances of complications as much as possible.
Side effects
It’s normal to have some cramping or spotting for a few hours after CVS, but you should call your provider ASAP if you experience heavy bleeding, fluid leaking from your vagina, fever or contractions. Let your provider know, too, if the cramping or spotting goes on for more than three days.
After your test, arrange for someone to drive you home if possible, and rest and relax for the rest of the day. You should also avoid exercise and sex. You can typically go back to your usual routines the next day, but do discuss any concerns with your doctor.
Emotionally, you might feel anxious, worried or just drained as you wait for your results — all the more reason to take it easy after the procedure. Sharing your concerns with your partner or a trusted family member or friend may help.
Finally, keep in mind that in most cases the results of a CVS test come back completely normal. But no matter what the outcome, you and your doctor can take time to talk about what to do next.
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