Chorionic villus sampling (CVS) is the removal of a small piece of tissue (chorionic villi) from the uterus during early pregnancy to screen the baby for genetic defects.

Depending on where the placenta is located, CVS can be performed through the cervix (transcervical) or through the abdomen (transabdominal). The techniques are thought to be equally safe and effective for obtaining samples. Both the transcervical and the transabdominal CVS are performed with ultrasound guidance.

An abdominal ultrasound is performed to determine the position of the uterus, the size of the gestational sac, and the position of the placenta within the uterus. Your vulva, vagina, and cervix are then cleansed with an antiseptic such as Betadine. For the transabdominal procedure, the abdomen is also cleansed.

The transcervical procedure is performed by inserting a thin plastic tube through the vagina and cervix to reach the placenta. Ultrasound is used to help guide the tube into the appropriate area and a small sample of chorionic villus tissue is withdrawn.

The transabdominal procedure is performed by inserting a needle through the abdomen and uterus and into the placenta. Ultrasound is used to help guide the needle, and a small amount of tissue is drawn into the syringe.

The sample is placed in a dish and evaluated in a laboratory.

Your health care provider will explain the procedure, its risks, and alternative procedures such as amniocentesis. Genetic counseling is recommended prior to the procedure. This will allow you to make an unhurried, informed decision regarding options for prenatal diagnosis.

You will be asked to sign a consent form before this procedure, and you may be asked to wear a hospital gown.

The morning of the procedure you may be asked to drink fluids and refrain from urinating to fill your bladder, which allows adequate visualization so the sample may be taken.

The ultrasound is not uncomfortable. A clear, water-based conducting gel is applied to the skin to help with the transmission of the sound waves. A handheld probe called a transducer is then moved over the area. In addition, your health care provider may apply pressure on your abdomen in an attempt to manually locate the position of your uterus.

The antiseptic cleansing solution will feel cold initially and may irritate your skin if it is not removed after the procedure. Some people are allergic to Betadine. Notify your health care provider if you are allergic to Betadine or if you have any other allergies.

Patients report that the actual sampling feels similar to a Pap smear, where there is some discomfort and a feeling of pressure. There may be a small amount of bleeding following the procedure.

An obstetrician can perform this procedure in about 5 minutes, after the preparation.