What is chorionic villus sampling?
Chorionic villus sampling (CVS), also known as chorionic villus biopsy, is a test performed during pregnancy to determine if an unborn child is at risk for congenital defects. During the procedure, the physician takes a sample of the chorionic villi. This is a tissue in the placenta that contains information about the baby’s genes. The genetic information in the tissue is then used to find out if an abnormal genetic or biochemical condition is present, such as Down syndrome, cystic fibrosis, or Tay-Sachs disease.
The test is performed fairly early in pregnancy. Some sources suggest CVS can be performed as early as eight weeks and as late as 13 weeks and six days. Between 10 and 12 weeks is the most common, according to Kathleen D. Pagana and Timothy J. Pagana, authors of “Mosby’s Diagnostic and Laboratory Test Reference.” Talk to your doctor about when the time is right for you.
Why do you need the test?
Not everyone who is pregnant will need to have CVS. It’s usually performed when there’s a possibility that the baby may have an abnormal condition.
Your doctor may consider CVS if:
- you’re over 35 years of age, since women over 35 are at higher risk of having children with chromosomal abnormalities
- you or your partner has a genetic disorder
- you or your partner has a family history of a genetic disorder
- you have had other children with genetic or biochemical disorders
- you have had spontaneous abortions
- you have had prenatal screening tests with abnormal results
The test can detect more than 200 different types of genetic and biochemical conditions. Performing the test early on during the pregnancy gives parents the information they need to cope with some of the complications of the pregnancy, or the possibility of terminating the pregnancy.
Are there any risks?
Some of the risks involved with the procedure include:
Bleeding and cramping
You may experience cramping or bleeding, especially if the procedure was performed through the cervix instead of the abdomen.
As with any invasive procedure, there is a risk of infection, though this complication occurs rarely.
During the procedure, there’s a chance that the baby’s blood may be mixed with the mother’s blood. If you have Rh-negative blood and your baby is Rh-positive, you may become sensitized. This will cause your body to produce antibodies that attack your baby’s blood cells.
If this occurs, your doctor can give you a medication called Rh immune globulin. This medication will stop you from becoming sensitized. If you are Rh-negative, make sure your doctor knows. If you’re not sure, ask your doctor.
Accidental abortion or miscarriage
The risk of miscarriage due to a CSV is small. According to the Mayo Clinic, chances are less than 1 in 100. The risk for miscarriage increases if the procedure is performed through the cervix instead of the abdomen. The risk also increases if the fetus is small for gestational age.
Fetal limb deformities
In some cases, chorionic villus sampling has caused deformities to the baby’s limbs, most notably the fingers and toes. However, this risk is low when the procedure is performed after nine weeks of pregnancy.
You should discuss the risks and benefits of the test with your doctor. Even though your doctor may recommend the test, undergoing the procedure is you and your partner’s decision.
How do you prepare for the test?
There are no food or fluid restrictions for this test. You may be told to drink one to two glasses of water or other fluids before the test begins. You will probably be asked to come with a full bladder because this will help with the ultrasound.
What happens during the test?
The test is usually performed by an obstetrician in the office. It usually takes about 30 minutes, and women report that it feels much like a Pap test. Before the procedure, you will sign a consent form, and have you and your baby’s vital signs taken.
The procedure can be performed through the cervix or through the abdomen. First, your doctor will ask you to lie on your back. Your doctor will then insert an endoscope with a syringe into you uterus. Using an ultrasound as a guide, your doctor will locate the placenta and use the syringe to take small samples of the villi. Your doctor may take two or three samples. When the procedure is complete, your doctor will take your vital signs again and check for any signs of bleeding.
You may have another ultrasound scheduled within two to four weeks to make sure your baby is doing well.
Understanding your test results
The results may not be available for several weeks. If your test is normal, that means there are no signs of a genetic defect. However, be aware that chorionic villus sampling does not test for every abnormal condition. In addition, some genetic abnormalities can be very difficult to identify and may not show up clearly during chromosome studies. If the test results are abnormal, your obstetrician will discuss the specifics with you. You may be referred to someone who specializes in genetic counseling.
The CVS test gives you the opportunity to learn about abnormal genetic or biochemical conditions that may be associated with your pregnancy, and consider what to do about them. Though there are risks involved in the test, the benefits may outweigh them if there is a possibility that your baby may have an abnormal condition. If you choose to have the test, be sure to discuss risks and possible complications with your doctor.