Chorionic villus sampling

Chorionic villus sampling (CVS) is a test carried out during pregnancy most commonly to check your baby for disorders such as Down’s syndrome and, where appropriate, rarer specific inherited disorders.

It involves removing and testing a sample of cells from the placenta (the organ linking the mother’s blood supply with her unborn baby’s).

CVS is offered in pregnancies where there is a high risk of the baby having such conditions. This could be because:

an earlier antenatal screening test has suggested that there may be a problem, such as Down's syndrome or sickle cell anaemia
you have had a previous pregnancy with these problems, such as a baby born with a chromosome abnormality
you have a family history of a condition such as cystic fibrosis or muscular dystrophy an abnormality is detected in your baby during the 11th to 14th week scan

If there is a risk of your child being born with one of these conditions, a specialist involved in carrying out the test will be able to discuss testing with you.

In some cases, you may be referred to a genetic counsellor (a healthcare professional trained in genetics). They will discuss your risk of passing on certain genetic conditions and can offer you advice about what to do when you get the results.

How CVS is performed

CVS is usually carried out between the 11th and 14th weeks of pregnancy, although it is sometimes performed later than this if necessary.

During the test, a sample of cells, called chorionic villi cells, will be taken from the placenta using one of two methods:

transabdominal CVS – a needle is inserted through the abdomen (this is the most common method used)
transcervical CVS – a tube or forceps (smooth metal instruments that look like large tongs) are inserted through the cervix (the neck of the womb)

The test itself takes about 10 minutes, although the whole consultation will take about 30 minutes.

The CVS procedure is uncomfortable rather than painful, although you may experience some cramps that are similar to menstrual cramps for a day or two afterwards.

Read more about what happens during CVS.

Getting the results

The first results of the test should be available within two to three working days and this will tell you whether a major chromosome condition, such as Down’s syndrome or Edward’s syndrome, has been discovered.

If rarer conditions are also being tested for, it can take two to three weeks to come back. If the test is looking for a specific disorder, it can take up to a month to get the results.

If your test shows that your baby has a serious inherited or genetic disorder, the implications will be fully discussed with you. There is no cure for most of the conditions tested for using CVS, so you will need to consider your options carefully.

You may choose to continue with your pregnancy, while gathering information about the condition so you are fully prepared, or you may consider having a termination (abortion).

What are the risks of CVS?

Before you decide to have CVS, the risks and possible complications will be discussed with you.

One of the main risks associated with CVS is miscarriage, which is the loss of the pregnancy in the first 23 weeks. This is estimated to occur in 1-2% of cases.

There are also some other risks, such as infection or needing to have the procedure again because it wasn't possible to test the first sample removed.

The risk of CVS causing complications is higher if it is carried out before the 10th week of pregnancy, which is why the test is only carried out after this point.

Read more about the possible complications of CVS.

What are the alternatives?

An alternative to CVS is a test called amniocentesis. This is where a sample of the mother's amniotic fluid is removed for testing. It is usually carried out between the 15th and 18th week of pregnancy, although it can be performed later than this if necessary.

This test may have a slightly smaller risk of causing a miscarriage, but your pregnancy will be at a more advanced stage before you can get the results and consider your options.

A termination should ideally be carried out as early in the pregnancy as possible. Under UK law, an abortion can usually only be carried out during the first 24 weeks of pregnancy. However, they can lawfully be carried out after this point if tests have shown the child will most likely be born with a serious abnormality or disability.

If you are offered tests to look for a genetic disorder in your baby, a specialist involved in carrying out the test will be able to discuss the different options with you and help you and your partner make a decision.