Chorionic villus sampling (CVS) is a prenatal test that involves taking a tiny tissue sample from outside the sac where the fetus develops. The tissue is tested to diagnose or rule out certain birth defects. The test generally is performed between 10 and 12 weeks after a woman's last menstrual period.
CVS is a diagnostic test. It is usually offered when there is an increased risk of chromosomal or genetic birth defects. Amniocentesis is another prenatal diagnostic test that can diagnose the same birth defects, but is performed a little later in pregnancy, usually between 15 and 20 weeks after a woman's last menstrual period. A woman may choose to have CVS if she would like test results earlier in pregnancy.
Diagnostic tests are different from screening tests. All pregnant women are offered screening tests to help evaluate their risk for certain birth defects, such as Down syndrome. Screening tests, however, cannot diagnose a birth defect.
Who is offered CVS?
CVS is not routinely offered to all pregnant women because the test carries a small risk of miscarriage.
CVS may be offered because of:
- Concern over first-trimester screening test results. The American College of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women be offered a screening test for Down syndrome, regardless of their age (1). Screening can be done in the first or second trimester. The first-trimester test, done at 11 to 13 weeks of pregnancy, consists of a maternal blood test and an ultrasound examination of the back of the baby's neck (nuchal translucency). These women are generally offered follow-up diagnostic testing with CVS or amniocentesis.
- Maternal age. Until recently, women over age 35 were routinely offered prenatal testing with CVS or amniocentesis because the risk of having a child with certain chromosomal birth defects increases as a woman ages. The most common of these disorders is Down syndrome, a combination of mental and physical abnormalities caused by the presence of an extra chromosome. The risk of Down syndrome increases from approximately 1 in 1,250 at age 25, to 1 in 1,000 at age 30, to 1 in 400 at age 35, 1 in 100 at age 40 and 1 in 30 at age 45 (2). Women over age 35 now have the option of having a screening test to evaluate their risk before deciding whether or not to proceed with CVS or amniocentesis. ACOG also recommends that pregnant women of all ages have the option of bypassing the screening test and choosing a diagnostic test (CVS or amniocentesis) (1).
- A previous child or pregnancy with a birth defect. A woman may be offered CVS or amniocentesis during subsequent pregnancies if she already has had a child or pregnancy diagnosed with a chromosomal abnormality or genetic birth defect.
- Other family history. Couples without a previously affected child also may be offered prenatal testing if their family medical histories indicate their children may be at increased risk of inheriting a genetic disorder. Prenatal testing is offered only if the suspected condition can be diagnosed before birth. Prenatal diagnosis is possible for virtually all chromosomal disorders, but not all genetic ones.
How is CVS performed?
CVS involves taking a small piece of the chorionic villi, which are wisps of tissue from the developing placenta, where it attaches to the wall of the uterus. A laboratory analyzes cells from the villi, which normally have the same genetic and biochemical makeup as the fetus.
There are two ways to perform CVS. The health care provider chooses the method that is best for the pregnant woman, depending on the position of the placenta.
- Transcervical CVS. The provider thoroughly cleanses the vagina and cervix with an antiseptic. Then, using ultrasound as a guide, the provider inserts a thin tube through the woman's vagina and cervix to the villi and uses gentle suction to remove a small sample.
- Transabdominal CVS. The provider cleanses the abdomen with antiseptic. Then, using ultrasound as a guide, the provider inserts a thin needle through the abdominal wall into the uterus. The needle is attached to a syringe, which is used to withdraw a small sample.
After the sample is taken, the provider checks the fetus's heartbeat with ultrasound. Most physicians recommend that a woman take it easy for several hours after CVS. Some women experience some vaginal bleeding or spotting and cramping for a few hours after CVS. A woman should contact her health care provider if she has heavy bleeding, a fever or uterine contractions.
Test results are generally available in about seven days, although preliminary results may be available sooner (3).
Is CVS painful?
Some women say CVS doesn't hurt at all. Others experience cramping, similar to menstrual cramps, when the sample is taken. Some women who have transcervical CVS report that it feels similar to having a Pap smear.
Is CVS safe?
Studies done in the 1980s, soon after CVS became available, suggested that CVS may be slightly more likely than amniocentesis to cause miscarriage (4). Women are often told by providers that CVS poses about a 1 percent (1 in 100) risk of miscarriage, compared to about 0.5 percent (1 in 200) for amniocentesis. However, as doctors have gained experience with CVS, the risk appears to have declined.
Recent studies suggest that CVS is no more likely than amniocentesis to cause miscarriage when performed by an experienced provider (4, 5). A 2006 study at the University of California, San Francisco, found that the rate of fetal loss was 1 in 360 for CVS compared to 1 in 370 for amniocentesis, a difference that is not statistically different (4).
The risk of pregnancy loss following CVS appears lower when the physician performing the procedure is highly experienced and when the testing facility provides both transcervical and transabdominal CVS (allowing the safer option for each patient). Studies suggest that transabdominal and transcervical CVS are equally safe (5).
In the early 1990s, safety concerns about CVS arose following several reports of babies born with missing or shortened fingers or toes, occasionally accompanied by abnormalities of the tongue and lower jaw. These birth defects were noted mainly when CVS was performed before the 10th week after the last menstrual period. Now CVS is routinely performed during the 10th or later weeks. More recent studies provide reassurance that there is no increased risk of limb defects when CVS is performed at 10 weeks or later (5).
Do good CVS results mean a baby will be born healthy?
Most women who have CVS or amniocentesis receive reassuring news that their unborn babies do not have the disorders for which they were tested. However, no prenatal test can guarantee the birth of a healthy baby, since only some birth defects can be ruled out before birth. Three to four out of every 100 babies have a birth defect (6).
CVS test results are very accurate (greater than 99 percent) in ruling out certain chromosomal birth defects and specific genetic problems. However, CVS is slightly more likely than amniocentesis to give inconclusive results, leaving questions that often can be resolved with amniocentesis.
Can doctors treat the birth defects diagnosed with CVS?
Currently, doctors are able to diagnose many more birth defects than they are able to treat before birth. However, advances in prenatal treatment now make it possible to treat some birth defects before birth.
For example, congenital adrenal hyperplasia (CAH), an inherited birth defect caused by the lack of an important enzyme, causes the female fetus to develop abnormal external genitals by the 16th week of pregnancy. Treating the fetus with hormones during the critical period of development from 10 to 16 weeks of gestation can prevent the need for surgery after birth. Now, in pregnancies at known risk for CAH, prenatal diagnosis using CVS can tell doctors whether this treatment is required.
If a fetus has a condition for which prenatal treatment is not possible, prenatal diagnosis may help parents to prepare emotionally for the birth and to plan the delivery with their health care providers. Parents can discuss their options with genetic counselors as well as with their health care providers.
Who should have CVS?
Whether or not to have prenatal diagnosis is a matter for discussion between parents and health professionals. Genetic counselors, physicians and religious and ethical counselors can be valuable in helping parents make informed decisions about prenatal testing, diagnosis, treatment and other reproductive issues. CVS is performed early in pregnancy, and the decision requires thoughtful discussion and careful planning. This is just one more reason why it is important that all women get early prenatal care, and why visiting a health care provider before conception is strongly recommended.
Couples deciding between CVS and amniocentesis need to consider many factors, including the technical expertise available, a woman's medical history and preferences, and what condition is being diagnosed. With one main exception (CVS cannot diagnose neural tube defects), CVS and amniocentesis test for the same birth defects. (Neural tube defects are birth defects of the brain and spinal cord.) The procedures differ slightly, however, in timing and how soon results are available.
Are there ways to reduce the risk of birth defects?
There are some basic things all women can do to improve their chances of having a healthy pregnancy and a healthy baby:
- Plan for pregnancy by seeing a health care provider before conception.
- Get early and regular prenatal care.
- Take a multivitamin containing 400 micrograms of folic acid daily starting before pregnancy and during early pregnancy to help prevent serious birth defects of the spinal cord and brain.
- Eat a variety of nutritious foods, including foods that are fortified with folic acid and foods that contain folate, the natural form of folic acid found in foods. Many grain products, including flour, rice, pasta, bread and cereals, are fortified with folic acid. Folate-rich foods include green leafy vegetables, dried beans, legumes, oranges and orange juice.
- Begin pregnancy at a healthy weight (not too heavy or too thin). Gain the recommended amount of weight during pregnancy (25 to 35 pounds for women who begin pregnancy at a normal weight).
- Don't drink alcohol during pregnancy.
- Don't smoke during pregnancy and avoid secondhand smoke.
- Don't use any drug, even over-the-counter or herbal medications, unless recommended by a health care provider who knows that you are pregnant.
- Don't eat undercooked meat or change a cat's litter box. Both can cause a parasitic infection called toxoplasmosis that can cause birth defects in the baby.
In addition, the March of Dimes recommends that all women be tested for immunity to rubella (German measles) and chickenpox before becoming pregnant. Consider being vaccinated if they are not immune. The vaccines are not recommended during pregnancy.
References
- American College of Obstetricians and Gynecologists (ACOG). Screening for Fetal Chromosomal Abnormalities. ACOG Practice Bulletin, number 77, January 2007.
- American College of Obstetricians and Gynecologists (ACOG). Your Pregnancy and Birth, 4th Edition. ACOG, Washington, DC, 2005.
- American College of Obstetricians and Gynecologists (ACOG). Birth Defects. Patient Education Pamphlet, number 1074-8601, April 2005.
- Caughey, A.B., et al. Chorionic Villus Sampling Compared with Amniocentesis and the Difference in the Rate of Pregnancy Loss. Obstetrics and Gynecology, volume 108, number 3, part 1, September 2006, pages 612-616.
- Evans, M.I. and Wapner, R.J. Invasive Prenatal Diagnostic Procedures 2005. Seminars in Perinatology, volume 29, 2005, pages 215-218.
- Centers for Disease Control and Prevention (CDC). Birth Defects: Frequently Asked Questions.
April 2007
