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

CVS can be done through the cervix (transcervical) or through the abdomen (transabdominal). The techniques are equally safe when done by a provider with experience, although miscarriage rates are slightly higher when done through the cervix. The health care provider will use ultrasound to pick the safest approach and as a guide during sampling.

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, cervix, and abdomen are cleaned with an antiseptic such as Betadine.

The transcervical procedure is performed by inserting a thin plastic tube through the vagina and cervix to reach the placenta. The provider uses ultrasound images to help guide the tube into the appropriate area and then removes a small sample of chorionic villus tissue.

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 doesn't hurt. 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 to find the position of your uterus.

The antiseptic cleansing solution will feel cold at first nd may irritate your skin if not washed off 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.

Some women say the vaginal approach feels like a Pap smear with some discomfort and a feeling of pressure. There may be a small amount of vaginal bleeding following the procedure.

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

The test is a way of detecting genetic disorders. The sample is used to study the DNA, chromosomes, and enzymes of the fetus. It can be done sooner than amniocentesis, about 10 to 12 weeks after your last menstrual period. Test results take about 1 to 2 weeks, whereas amniocentesis results may take longer.

Chorionic villus sampling does not detect neural tube defects. If neural tube defects or Rh incompatibility are a concern, an amniocentesis will be performed.

This test can usually not diagnose problems in the way the body forms.

A normal result means there are no signs of any genetic defects. However the test could miss some genetic defects.

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

An abnormal result may be a sign of more than 200 disorders, including:

• Down syndrome
• Hemoglobinopathies
• Tay-Sachs disease

The risks of CVS are only slightly higher than those of an amniocentesis.

Possible complications include:

• Bleeding
• Infection
• Miscarriage
• Rh incompatibility in the mother
• Rupture of membranes

Signs of complications include:

• Excessive bleeding
• Excessive vaginal discharge
• Fever

Report any signs of complications to your health care provider.

CVS may also cause limb problems in the fetus. This risk appears to be very low (1 in 3,000) when CVS is performed after 10 weeks gestational age.

If your blood is Rh negative, you may receive RhoGAM to prevent Rh incompatibility.

You will receive a follow-up ultrasound 2 to 4 days after the procedure to make sure the pregnancy is proceeding normally.

Simpson JL. Otano L. Prenatal Genetic Diagnosis. In: Gabbe SG, Niebyl JR, Simpson JL. Obstetrics - Normal and Problem Pregnancies. 5th ed. New York, NY: Churchill Livingstone; 2007: Chap. 7.