Chorionic villus sampling

Definition

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.

Why the Chorionic villus sampling is Performed

This test is a way of detecting congenital (present from before birth) defects. The sample is used to study the DNA, chromosomes, and enzymes of the fetus and can be done earlier than amniocentesis, about 10 to 12 weeks after your last menstrual period.

Chorionic villus sampling results from a CVS are obtained within 1 to 2 weeks, whereas amniocentesis results may take as long as a month. Earlier diagnosis of congenital defects may relieve some of the concerns of your family, especially if a therapeutic abortion is under consideration.

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

How the Chorionic villus sampling is Performed

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 Ultrasound is used to help guide the tube into the appropriate area and a small sample of chorionic villus tissue is withdrawn.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.

How to Prepare for the Chorionic villus sampling

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.

How the Chorionic villus sampling Will Feel

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.

Risks

The risks of CVS are only slightly higher than those of an amniocentesis. Fetal loss occurs about 2% of the time.

Possible complications include the following:

• Rupture of membranes (rupture of the amniotic membrane)
• Spontaneous abortion (miscarriage)
• Infection
• Bleeding
• Rh incompatibility in the mother
• Contamination of the sample with maternal cells

Considerations

• CVS may be associated with an increased risk for limb anomalies in the fetus. This risk appears to be very low (1 in 3,000) when CVS is performed after 10 weeks gestational age.
• After this test, if your blood is Rh negative, you may receive RhoGAM to avoid Rh incompatibility.
• Two to 4 days after the CVS, you will be scheduled for a follow-up ultrasound, to ensure that the fetus survived the procedure.

Report any signs that there may be complications of the procedure to your health care provider:

• Excessive bleeding
• Excessive vaginal discharge
• Fever
• Miscarriage

Normal Results

Normal values would indicate an absence of congenital defects.

What Abnormal Results Mean

Analysis of the biopsy may indicate any of more than 200 disorders, including the following:

• Down syndrome
• Hemoglobinopathies
• Tay-Sachs disease

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