Chorionic villus sampling (CVS) is a relatively new procedure used to diagnose certain birth
trimester of pregnancy. The test has been performed
regularly since 1982, and many thousand have been performed around the world. CVS is a prenatal test that involves taking a tiny tissue sample from outside the sac where the fetus develops.
This tissue contains the same genetic material as the fetus.
The chorion is the portion of fetal membrane that eventually forms
the fetal side of the placenta. The chorion contains villi which are small finger-like projections. These villi are snipped or suctioned off for study in the procedure. The chorionic villi are of fetal origin so examining the samples of them can give the genetic makeup of the fetus. This test is performed as an early detection tool of congenital defects.
The test generally is performed between 10 and 12 weeks after a woman‚Äôs last menstrual period. CVS usually is not recommended if a woman has bleeding or spotting during the pregnancy.
How is CVS Performed?
CVS requires taking a small piece of the chorionic villi, which are wisps of tissue that attach the pregnancy sac 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. Test results are generally available in about 10 days, although preliminary results may be available sooner.
How is the Sample Taken?
First, the vagina and cervix are thoroughly cleansed with an antiseptic. Then, using ultrasound as a guide, a physician inserts a thin tube through the vagina and cervix (transcervical CVS) to the villi, and uses gentle suction to remove a small sample. No anesthetic is required. Some women say CVS doesn't hurt at all; others experience cramping or a pinch when the sample is taken.
Depending upon an individual woman’s anatomy, the physician may choose to reach the chorionic villi by inserting a needle through the abdominal wall (transabdominal CVS), also using ultrasound guidance. Studies have found the two forms of CVS to be equally safe, unless the woman has a retroverted (tipped) uterus, in which case the risk of miscarriage is higher if the procedure is done transcervically. Therefore, transabdominal CVS is recommended for women with a retroverted uterus. If the location of the placenta prevents this procedure, amniocentesis can be considered as an alternative.
After the sample is taken, the fetus's heartbeat is checked with ultrasound before the woman leaves the examination room. Most physicians recommend that a woman take it easy for several hours after CVS. One in five women experience cramping following CVS; 1 in 3 women have some bleeding or spotting, which ordinarily stops within a few days. A woman always should report these symptoms to her health care provider.
Is CVS Safe?
More than 200,000 women worldwide have undergone CVS, which was introduced in this country in 1983. Studies suggest that CVS may be slightly more likely than amniocentesis to cause miscarriage. According to the Centers for Disease Control and Prevention, between 1 in 200 and 1 in 100 women miscarry after CVS. That risk rises to about 5 in 100 for a woman with a retroverted uterus who has transcervical CVS. The risk of pregnancy loss following CVS is 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). Recent studies suggest that the rate of miscarriage following CVS and amniocentesis (at 15 to 18 weeks) may be about the same with experienced doctors in such facilities.
CVS test results are very accurate (greater than 99 percent) in ruling out certain chromosomal birth defects and specific genetic problems.