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What is chorionic villi sampling?
Chorionic villi sampling (CVS) is a prenatal test that can detect genetic abnormalities such as Down's syndrome. Unlike amniocentesis, CVS does not screen for neural tube defects. It is performed earlier than amniocentesis - between the 10th and 12th weeks of pregnancy. In CVS, a catheter or thin needle is inserted into the womb to extract some of the chorionic villi - cells from the tissue that will become the placenta. The chorionic villi contain the same chromosomes as the fetus.
Why would I have this test?
You may want to have the CVS test if there is an increased risk of chromosomal or genetic birth defects, and if you would like test results as early in pregnancy as possible. The test is not routinely offered to all pregnant women because the test carries a small risk of miscarriage, and possibly other complications. Also, CVS usually isn't recommended if a woman has bleeding or spotting during the pregnancy.
CVS may be offered because of:
- Advanced maternal age. The risk of bearing a child with certain chromosomal birth defects increases as a woman ages. Prenatal testing is offered if a woman will be 35 or older at the time of delivery.
- 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 chromosomal abnormality or genetic birth defects diagnosed in a prior pregnancy or child.
- Family history that indicates the baby may be at increased risk of inheriting a genetic disorder. Prenatal diagnosis is possible for virtually all chromosomal disorders, but not all genetic ones.
- An abnormal nuchal translucency (NT), a type of ultrasound exam.
How is CVS performed?
In a private exam room, your vagina and cervix are thoroughly cleansed with an antiseptic. Then, with an ultrasound as a guide, your doctor inserts a thin tube through your vagina and cervix (transcervical CVS) to the villi, and uses gentle suction to remove a small sample. No anesthetic is required.
The physician may choose an alternate method to reach the chorionic villi by inserting a needle through the abdominal wall (transabdominal CVS), also using ultrasound guidance. Both forms of CVS have found to be equally safe, although the transabdominal CVS is recommended for women with a tipped uterus because the risk of miscarriage is higher if done transcervically.
After the procedure, the fetal heartbeat is checked with ultrasound before you leave the examination room. You may experience lower abdominal cramping following CVS, or some bleeding or spotting, which ordinarily stops within a few days. If you have these symptoms, tell your doctor.
Are there any risks associated with CVS?
Studies suggest CVS may be slightly more likely to cause miscarriage than amniocentesis. For women with an anteverted uterus, there is less of an increased risk of miscarriage. Complication rates from CVS have been shown to be lower among physicians who perform this test more frequently. Before consenting to this test, check with your doctor and determine how often he/she has performed this test.
There has been some concern that the test itself may be associated with limb deformities. However, many geneticists believe that CVS performed between the 10th and 12th weeks of pregnancy does not increase that risk.
How accurate are the results?
Test results have a greater than 99 percent accuracy rate in ruling out certain chromosomal birth defects and specific genetic problems. However, the test is slightly more likely than amniocentesis to give inconclusive results.
Does a negative CVS result mean a baby will be born healthy?
More than 95 percent of the high-risk women who have any prenatal diagnosis receive reassuring news that their unborn babies do not have a disorder for which they were tested. However, no prenatal test can guarantee a healthy baby, since only some birth defects can be ruled out before birth. Three to four of every 100 babies have a birth defect.
Can birth defects diagnosed with CVS be treated?
Advances in prenatal treatment make it possible to treat some birth defects before birth. However, physicians are able to diagnose more birth defects than they are able to treat prenatally. If a fetus has a condition for which prenatal treatment is not possible, prenatal diagnosis may help parents to decide whether to continue the pregnancy or prepare emotionally for the birth.
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