Chorionic villi sampling (CVS) is a procedure performed in the first trimester of pregnancy to detect to detect birth defects by taking a sample of cells from parts of the placenta called the chorionic villi, which have the same genes as the fetus. CVS can detect most of the same defects as amniocentesis, but it cannot detect open neural tube defects. If you have CVS, you will want to consider having a blood alpha-fetoprotein (AFP) test later in your pregnancy to test for neural tube defects.

Here is some information about the test and the risks involved.

Who Should Have CVS?

Your doctor will discuss the benefits and risks of the test with you based on your particular pregnancy. This test is only useful if the results will change the management of your pregnancy, or change your desire to continue the pregnancy. If you are at higher risk for having a baby with birth defects, you may wish to have this test. Examples of things that may put you at higher risk are:

• Age 35 years old or older when you give birth.
• Family history of certain birth defects.
• Previous child with a birth defect.
• Abnormal results on other genetic screening tests.

Understanding the Procedure

CVS is usually done about 10-12 weeks from a woman's last menstrual period and once the presence of a living pregnancy has been established. The procedure is performed in the doctor's office or hospital. Cells can be collected from the placenta in two ways—through the abdomen or through the vagina. If you have any bleeding during pregnancy, problems with your cervix, or a sexually transmitted disease, you may be offered CVS through the abdomen as the preferred route.

Your doctor will do an ultrasound exam to determine the baby's age and the position of the placenta. This will help determine whether cells are collected through the abdomen or through the vagina. If collecting cells through the abdomen, the clinician will carefully insert a needle through your abdomen into your uterus and into your placenta under local anesthesia. A sample of chorionic villi will be collected. If collecting cells through the vagina, the clinician will first insert a speculum. A thin tube will be inserted into your vagina and up through your cervix. An ultrasound will be used to guide the tube to your placenta and a small sample of chorionic villi will be removed and sent to a lab. Results may take up to two weeks.

Risks of Having CVS

There is a risk of infection with CVS. Because the procedure is done earlier than amniocentesis, there is a slightly higher risk of miscarriage. You may also experience cramping or bleeding. In rare cases, limb deformities have occurred in infants, especially when CVS was done before nine weeks.

Not every woman has this test with every pregnancy. If you feel you fall into one of the categories listed above, or you have other concerns about prenatal testing, talk to your doctor.