What is chorionic villus sampling?
Chorionic villus sampling, more commonly called CVS, is a prenatal test used to identify birth defects and disorders. CVS is usually performed 10 to 12 weeks after your last menstrual period.
Chorionic villi are microscopic, finger-like wisps of placental tissue formed from your fertilized egg. Because villi cells normally have the same genetic material as your baby, tests using these cells can confirm or rule out certain genetic problems.
This is not a routine prenatal procedure, mainly because it carries a small risk of miscarriage and other complications. Your doctor will recommend the test only if your baby is at increased risk for genetic disorders. It will probably be offered to you if non-invasive screening for genetic disorders (consisting of a blood test and ultrasound) reveals that you are at risk. But regardless of these factors, whether you choose to have the test is up to you and your partner.
What does the test detect?
The procedure can identify more than 200 birth defects and disorders. These include abnormalities such as Down syndrome, a genetic condition caused by an extra chromosome marked by some degree of mental retardation and other disorders.
This test can also uncover inherited disorders, such as Tay-Sachs disease, which can cause mental retardation and blindness; sickle-cell anemia, a painful blood disorder; and cystic fibrosis, a chronic, debilitating lung condition.
Women 35 and older are at higher risk of having a baby with Down syndrome or other genetic disorders, as are women and their partners with a prior history of genetic disorders or a child with a genetic birth defect. However, the American Congress of Obstetricians and Gynecologists now recommends that non-invasive blood screening for genetic disorders be offered to all women, regardless of age. If the results suggest that you are at risk, ACOG recommends offering diagnostic testing, which includes invasive procedures like CVS and amniocentesis.
How should I prepare for the test?
Before having the procedure, you'll want to meet with your health-care practitioner to learn more about CVS and discuss the benefits and risks of the test. Along with your doctor or midwife, you may also want to speak with a genetic counselor or spiritual advisor. Getting answers to your questions can help you and your partner make an informed decision about whether to have the test.
How is the test performed?
First, your practitioner will use ultrasound to get a good look at your baby and the surrounding structures. This will help the practitioner plan the procedure.
The test is basically a biopsy of the developing placenta. There are two ways to do the procedure -- the position of the placenta frequently determines which method is more appropriate. In the transcervical CVS method, your vagina and cervix are cleansed with antiseptic and then a speculum (the metal instrument used when you have a Pap smear) is inserted into your vagina. The technician then uses ultrasound to guide a thin tube through your cervix to the placenta. The chorionic villi cells are gently suctioned into the tube.
Transcervical CVS is not appropriate if you have uterine fibroids or a tilted or tipped (retroverted) uterus, both of which can interfere with the path of the catheter. The procedure is also not recommended for women with a cervical infection, such as a sexually transmitted disease, or those who have experienced vaginal bleeding during pregnancy.
The second method is called transabdominal CVS and is similar to amniocentesis. In this procedure, you'll receive a local anesthetic at the puncture site then an ultrasound guides a needle through your abdomen to your placenta, and a thin needle draws a sample of tissue.
Neither transcervical nor transabdominal CVS is recommended for women who have experienced vaginal bleeding during pregnancy.
After preparation, both CVS procedures take about five minutes. The cell samples are then placed in a dish and sent to a lab where the DNA, chromosomes, and enzymes are analyzed under a microscope. Preliminary results may be available in a few days, though it typically takes about 7 days for complete information.
Some women feel no pain with either type of CVS, but others say they experience the same pinching or cramping sensation they get after having a Pap smear. You may have light bleeding, or spotting, following either test. Your practitioner may advise you to rest after the procedure, but you can usually resume regular activities by the following day.
What are the risks of CVS?
CVS is considered a safe procedure for pregnant women, but it is an invasive test and it does pose risks. Miscarriage is the primary concern: There is a 1 percent chance (one in a 100) of having a miscarriage as a result of undergoing CVS, according to the American Congress of Obstetricians and Gynecologists.
Another concern is the slight chance that CVS will allow your and your baby's blood to mix. If your baby has a protein in her blood called Rhesus (Rh) factor (making her Rh positive) and you don't (making you Rh negative), your body may produce antibodies to fight your baby's blood. To prevent this from happening, your doctor will give you an injection of a drug called Rh immune globulin which prevents your body from recognizing Rh positive blood.
Some studies suggest that in a small number of cases -- about one in 3,000 -- CVS may cause defects in the fingers or toes of developing babies. Other research has found no increased risk of these limb abnormalities in women who undergo CVS between 10 and 12 weeks of pregnancy, which is the standard in the United States. In scientific journals there is some debate over whether these anomalies occur more often if CVS is performed on or before the ninth week of pregnancy, so talk with your doctor about timing.
Side effects are also possible. After the test you may experience spotting, cramping, pain, infection, fever, chills, or leaking of amniotic fluid. Light spotting and cramping is usually not a cause for alarm, though you should tell your health-care provider about these symptoms. If you experience these or any other problems following the procedure, call your doctor or midwife immediately.
What do the results mean?
CVS is a highly accurate diagnostic test. And most women -- more than 95 percent of the high-risk women who have the procedure -- get good news: Their unborn babies do not have the disorders for which they are tested.
Still, no test is 100 percent perfect, and not all birth defects can be ruled out before birth. In addition, CVS does not measure the severity of the disorders it identifies.
And as with any diagnostic procedure, there is a small chance of getting a false-positive result, meaning that the test indicates your baby has an abnormality when she does not. However, the chance of getting a false-positive result with CVS is only about one percent.
What happens after the test?
If your test reveals abnormalities, discuss your options with your health-care provider. Any decision you make is a personal one, but you'll want to have all the information you need to make an informed choice. Equipped with this knowledge you can pursue any interventions that may help, start preparing for a child with special needs, begin assessing any changes in lifestyle, seek out support groups and resources, or decide not to continue your pregnancy.
On occasion, unclear CVS results may require further blood work from parents, ultrasound, a repeat CVS, fetal blood sampling, or amniocentesis at 15 to 16 weeks to clarify the results.
What are the pros and cons of CVS versus amniocentesis?
Risk, timing, availability, and medical appropriateness are all factors to consider in choosing one test over the other.
If you have results from non-invasive tests indicating that your fetus may have genetic birth defect, CVS can be done earlier than amniocentesis, which is typically conducted between the 15th and 20th weeks of pregnancy. CVS also provides results more quickly than amnio, which can take about two weeks. Some women prefer to know results to genetic tests in their first trimester, when it's safer to terminate a pregnancy.
But unlike amnio, CVS cannot detect a category of abnormalities known as neural tube defects. Neural tube defects, such as spina bifida, occur when the brain and spinal cord don't develop properly. If you have a genetic blood-screening test, such as the alpha-fetoprotein test, or AFP, and your results suggest you're at greater risk of having a child with a neural tube defect, or if you have already had a child with this condition, then amniocentesis may be a better choice for you. If you and your doctor decide on CVS, you should get an AFP test at 16 weeks.
In addition, some studies show that amniocentesis has a slightly lower rate of miscarriage than CVS. While the miscarriage rate is about one in 100 for CVS, the risk of miscarriage from amnio is between one in 500 and one in 300, according to the March of Dimes, although one study found the miscarriage rate following amnio to be only one in 1,600.
The risk from CVS is thought to be lower if the procedure is done by a highly experienced physician in a facility that offers both transcervical and transabdominal CVS, allowing you to choose the option that is safer for your body. Compared with amniocentesis, CVS is a newer procedure and may not be performed at all hospitals or medical centers.
When weighing your choices, you'll want to consider the experience of the health-care professionals available to you, your medical history, your partner's medical history, and the conditions being diagnosed.
Deciding which test to take and waiting for the results can be a worrisome, anxiety-provoking experience. That said, it may help to remember that the vast majority of women at high risk of carrying a baby with a birth defect end up giving birth to a perfectly healthy child.
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ACOG Issues New Guidelines for Down Syndrome Testing, Genetics and Public Policy Center, February 2007.
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