Originally published by Harvard Health.
What is the test?
Chorionic villi are small structures in the placenta that act like blood vessels. These structures contain cells from the developing fetus. A test that removes a sample of these cells through a needle is called chorionic villus sampling (CVS).
CVS answers many of the same questions as amniocentesis about diseases that the baby might have. Diseases that can be diagnosed with CVS include Tay-Sachs, sickle cell anemia, cystic fibrosis, thalassemia, and Down syndrome. (Rh incompatibility and neural tube defects, however, can be diagnosed only through amniocentesis.) CVS can be done earlier in pregnancy than amniocentesis and can be done when there is not enough amniotic fluid to allow amniocentesis. However, it has some extra risks when compared with amniocentesis.
How do I prepare for the test?
CVS can be done between the 10th and 13th weeks of pregnancy. Tell your doctor ahead of time if you have ever had an allergic reaction to lidocaine or the numbing medicine used at the dentist’s office.
What happens when the test is performed?
There are two ways that your doctor can perform CVS. Some patients have the sampling done through the vagina and cervix. Most patients have the sampling done through the abdominal wall. For both types of sampling, you lie on your back on an examination table and the doctor uses ultrasound to locate the fetus and the placenta.
If the sampling is to be done through the vagina and cervix, you place your feet in footrests and bend your knees up, as you would for a pelvic examination. A speculum (a device that looks like a duck-bill that can be opened and closed) is used to open the vagina so that your doctor can see inside. A long tube, much narrower than a straw, is inserted through the cervix and moved forward while your doctor watches on the ultrasound until it is next to the fetal side of the placenta. A small sample of the lining around the fetus is then pulled into the tube for testing.
If the sampling is to be done through the abdominal wall, your lower abdomen is cleaned with an antibacterial soap. In some cases, the doctor uses a small needle to inject a numbing medicine just under the skin, so that you do not feel the sampling needle. (Because the sampling needle does not cause much more stinging than the numbing medicine itself, not every doctor includes this step.) A hollow needle several inches long is inserted through the skin and muscle of the abdomen and through the wall of the uterus, to the edge of the placenta. This needle is held in place as a guide needle. A narrower needle is then inserted through the first needle and is rotated and moved inward and outward a number of times while a sample is collected into an attached syringe.
The fetal heart tones and the mother’s blood pressure and heart rate are checked at the beginning and end of the procedure. The whole procedure takes close to 30 minutes.
What risks are there from the test?
The risk of miscarriage and other complications from CVS is slightly higher than the risk from amniocentesis, although some parents feel that it is worth the extra risk to be able to make decisions earlier in the pregnancy if the results show the baby has a health problem. There have also been some reports that suggest there is a very small risk of birth defects (abnormal limbs) in the fetus.
One particular difficulty with this test is that due to variability in the cells of the placenta (called mosaicism); occasionally you can have an abnormal test result even if the baby is normal and healthy. This might lead you to make decisions about pregnancy termination that you would not have made if you had better information.
Some women have vaginal bleeding after the procedure. Infection is uncommon.
Must I do anything special after the test is over?
You should notify your doctor if you are having vaginal bleeding, especially if it is heavy or lasts longer than three days. You should also tell your doctor about strong abdominal pain or fever. Doctors usually recommend avoiding exercise and sex on the day you have had the procedure.
How long is it before the result of the test is known?
Chromosome analysis of the sample takes two weeks or more. The results of some tests may be available sooner.