Family doctor


Pregnancy And Birth



CVS is a test on the placenta to check for possible genetic abnormalities in the fetus. This article looks at how the test is performed and the risks of the procedure.

What are chorionic villi?

Chorionic villi are part of the developing placenta and, as such, are fetal cells. By obtaining some of these cells early in pregnancy, it is possible to assess the chromosomal make up of the fetus and, on occasions, to identify whether particular infants are likely to carry deleterious genes.

The advantage of this test over amniocentesis is that it can be done earlier and, consequently, results can be obtained earlier.

The test is usually done at 11-12 weeks. It has been performed worldwide since 1983.

There are two main techniques of taking the sample:

1. Vaginal or transcervical sampling.

2. Transabdominal sampling.

Transabdominal sampling will normally be used at Fertility Associates.

Abdominal Chorionic Villus Sampling

This is a very similar procedure to amniocentesis, except that the amniotic sac is not entered. Local anaesthetic is inserted into the skin of the abdominal wall. Under ultrasound control, the developing placenta is identified and a needle is inserted into the wall of the uterus. The same needle is then inserted into the placenta. When the needle is in the placenta, a syringe is attached and a sample of cells is removed by suction.

The Risks

The procedure is associated with a risk of miscarriage of approximately 1/2 - 1% on top of the usual risk of miscarriage in any pregnancy. This is similar to amniocentesis. As the needle is not normally inserted into the amniotic sac or into the fetus, the risks of fetal damage from the procedure are very small. On rare occasions, it may be necessary to do what is essentially a very early amniocentesis by taking fluid from the sac. Although this is usually a simple test, some women find it uncomfortable and very occasionally, painful.

For this reason, it is good if you can have your husband/partner/support person with you at the time of the procedure. Sometimes more than one attempt may need to be made before suitable tissue is obtained. Occasionally, the procedure is unsuccessful. If this occurs, a further attempt might be made a week or so later or amniocentesis might be necessary.

A few years ago evidence showed that, if CVS is done prior to 9 1/2 weeks, some abnormalities in fetal limb development may rarely occur. Because of this, if there is any doubt about gestation, then the procedure may be deferred until you are definitely more than ten weeks gestation.


Results take two to three weeks for chromosome analysis. The chances of a confusing result, due to contamination with maternal cells, is slightly higher than with amniocentesis and an amniocentesis might be required to check the diagnosis in 1% to 2% of cases. Very rarely, a confusing result might be obtained which will require further elucidation. Rarely, the cell culture technique fails and no result can be obtained. The procedure can be repeated if this occurs.

Request Form for this Procedure

You will be asked to sign a request form for this procedure which states that you have read this information and understand it. If you do not understand anything, please discuss this further with the staff at the clinic.

See also:

Did this article meet your requirements/expectations?