Chorion villus sampling (CVS) Prenatal diagnosis and testing

The chorion is the tissue that surrounds the foetus during early pregnancy, part of which later becomes the placenta (afterbirth). It contains the same DNA as the foetus. A tiny sample of the chorion is obtained at a very early stage of the pregnancy.

Scientists will use 'direct genetic tests' or 'linked genetic markers' (see our factsheet on Carrier testing and options for reproduction) to try and discover whether or not the foetus has inherited the genetic mutation. They will be looking for the same genetic mutation as the one already identified in an affected person in the family. The parents will be told the results, and will then decide whether or not to continue with the pregnancy.

What does the test involve?

The CVS test can be performed at about 10 to 12 weeks of pregnancy. An ultrasound scan is performed first, to check how far advanced the pregnancy is, and to determine the position of the placenta. The test involves passing a needle through the abdomen in a similar way to an amniocentesis (see below) to obtain a small piece from the chorion. The test usually takes about 20 to 30 minutes, and can be done in a hospital outpatient department. The woman being tested is awake and aware of what is happening during the test.

Is the test painful?

Most women describe the test as being uncomfortable rather than painful. Local anaesthetic is usually used but most women say that they are aware of a 'pushing' feeling; they may feel some soreness over the area afterwards.

How accurate is the test?

Sometimes it is important to know the sex of the foetus and this can be determined very quickly using this test. Identifying a genetic mutation can depend on several factors, such as the size of the mutation, and therefore how easy it is to find, or whether or not a sample from an affected relative has been obtained, or how accurate 'linked genetic markers' are. Your geneticist or obstetrician can discuss all these factors with you before any test, and explain whether or not there is a risk of error with the test result.

Is the test safe?

There is a very small risk of miscarriage (one to two percent), which is a higher risk than for amniocentesis (see below). As far as the mother's health is concerned, there are thought to be no serious implications, although she should avoid strenuous exercise for a few days following the test.