What is chorionic villus sampling?
Chorionic villus sampling (CVS) is a prenatal test that involves taking a sample of tissue from the placenta to test for chromosomal abnormalities and certain other genetic problems. The placenta is a structure in the uterus that provides blood and nutrients from the mother to the fetus.
The chorionic villi are tiny projections of placental tissue that look like fingers and contain the same genetic material as the fetus. Testing may be available for other genetic defects and disorders depending on the family history and availability of lab testing at the time of the procedure.
CVS is usually done between the 10th and 13th weeks of pregnancy. Unlike amniocentesis (another type of invasive prenatal test), CVS does not provide information on neural tube defects, such as spina bifida. For this reason, women who undergo CVS also need a follow-up blood test between 16 to 18 weeks of their pregnancy to screen for neural tube defects.
There are two types of CVS procedures:
- Transcervical. In this procedure, a catheter is inserted through the cervix into the placenta to obtain the tissue sample
- Transabdominal. In this procedure, a needle is inserted through the abdomen and uterus into the placenta to obtain the tissue sample
Another related procedure that may be used to diagnose genetic and chromosomal defects is amniocentesis.
Reasons for the procedure
Chorionic villus sampling may be used for genetic and chromosome testing in the first trimester of pregnancy . Here are some reasons that a woman might elect to undergo CVS:
- Previously affected child or a family history of a genetic disease, chromosomal abnormalities, or metabolic disorder
- Maternal age over 35 years by the pregnancy due date
- Risk of a sex-linked genetic disease
- Previous ultrasound with questionable or abnormal findings
- Abnormal cell-free fetal DNA test
- There may be other reasons for your doctor to recommend a chorionic villus sampling.
Risks of the procedure
- As with any invasive procedure, complications may occur. Some possible complications may include, but are not limited to, the following:
- Cramping, bleeding, or leaking of amniotic fluid (water breaking)
- Infection
- Miscarriage
- Preterm labor
- Limb defects in infants, especially in CVS procedures done before 9 weeks (rare)
Women with twins or other multiples will need sampling from each placenta in order to study each baby.
Certain factors or conditions may interfere with CVS test results. These factors include, but are not limited to, the following:
- Position of the baby, placenta, or position of uterus/cervix
- Vaginal or cervical infection
- Samples that are inadequate for testing, or that may contain maternal tissue
There may be other risks depending upon your specific medical condition. Be sure to discuss any concerns with your doctor prior to the procedure.
Before the procedure
The doctor will explain the procedure to you and offer you the opportunity to ask any questions that you might have about the procedure.
You will be asked to sign a consent form that gives your permission to do the procedure. Read the form carefully and ask questions if information is not clear.
Generally, there is no special restriction on diet or activity prior to chorionic villus sampling.
Tell your doctor if you are sensitive to or are allergic to any medications, latex, iodine, tape, and anesthetic agents (local and general).
Tell your doctor of all medications (prescribed and over-the-counter) and herbal supplements that you are taking.
Tell your doctor if you have a history of bleeding disorders or if you are taking any anticoagulant (blood-thinning) medications, aspirin, or any other medications that may affect blood clotting. It may be necessary for you to stop these medications prior to the procedure.
Tell your doctor if you are Rh negative. During the CVS procedure, blood cells from the mother and fetus can mix. This may lead to Rh sensitization and breaking down of fetal red blood cells. In most cases, prenatal blood tests will have determined whether you are Rh negative. You may be asked to provide these lab results before the procedure.
You may or may not be asked to have a full bladder right before the procedure. Depending on the position of the uterus and placenta, a full or empty bladder may help move the uterus into a better position for the procedure.
Based upon your medical condition, your doctor may request other specific preparation.
During the procedure
Generally, a CVS procedure follows this process:
- An ultrasound will be performed to check the fetal heart rate, and the position of the placenta, fetus, and umbilical cord.
- Based on the location of the placenta, the CVS procedure will be performed through your cervix (transcervical) or through your abdominal wall (transabdominal).
- You will be asked to lie down on an exam table and place your hands behind your head.
For a transcervical CVS procedure:
- You will be asked to undress completely, or from the waist down, and put on a hospital gown.
- The doctor will insert an instrument called a speculum into your vagina so that he or she can see your cervix.
- Your vagina and cervix will be cleansed with an antiseptic solution.
- Using ultrasound guidance, a thin tube will be guided through the cervix into the placenta.
- Cells will be gently suctioned through the tube into a syringe. You may feel a twinge or slight cramping. More than one sample may be needed to obtain enough tissue for testing.
- The tube will then be removed.
For a transabdominal CVS procedure:
- For an abdominal CVS, your abdomen will be cleansed with an antiseptic. You will be instructed not to touch the sterile area on your abdomen during the procedure.
- The doctor may inject a local anesthetic to numb the skin. If a local anesthetic is used, you will feel a needle stick when the anesthetic is injected. This may cause a brief stinging sensation.
- Ultrasound will be used to help guide a long, thin, hollow needle through your abdomen and into the uterus and placenta. This may be slightly painful, and you may feel a cramp as the needle enters the uterus.
- Cells will be gently suctioned into a syringe. More than one sample may be needed to obtain enough tissue for testing.
- The needle will then be removed. An adhesive bandage will be placed over the abdominal needle insertion site.
After the procedure, both methods:
- The fetus’ heart rate will be reassessed.
- If you are Rh negative, you may be given Rho(D) immune globulin. This is a specially developed blood product that can prevent an Rh negative mother’s antibodies from reacting to Rh positive fetal cells.
- The chorionic villus tissue will be sent to the lab.
Your doctor will give you post-procedure instructions.
After the procedure
The CVS tissue will be sent to a specialty genetics lab for analysis. Counseling with a genetics specialist may be recommended depending on the test results.