Chorionic villus sampling (CVS) is a test carried out during pregnancy to detect specific abnormalities in an unborn baby. A sample of cells is taken from the placenta (the organ that links the mother’s blood supply with her unborn baby’s) and tested for genetic defects.
CVS is offered in pregnancies where there is a high risk of the baby having a serious inherited condition. This could be because:
Read more about why CVS is used, including information on other conditions it can help to diagnose early.
CVS is carried out between weeks 10 and 13 of pregnancy and is not recommended earlier than this. The risk of CVS causing complications, such as miscarriage or birth defects in the baby, is higher if it is carried out before week 10 of the pregnancy.
Read more about the possible complications of CVS.
CVS or amniocentesis?
CVS is an alternative to amniocentesis, where a sample of the mother's amniotic fluid is taken for testing. CVS can be carried out earlier than amniocentesis, which is usually carried out between weeks 15 and 20 of pregnancy.
Results from amniocentesis can take two to three weeks to come through. This may mean that your pregnancy is at a more advanced stage, around 20 weeks or more, before you can consider the results.
If you are at risk of passing a genetic condition onto your child, your GP or midwife will be able to discuss the tests with you, explain why they might be necessary, and help you and your partner make your decision.
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 of CVS.
How is it performed?
During CVS, a sample of cells, called chorionic villi cells, will be taken from the pregnant woman’s placenta using either:
- transabdominal CVS – a needle is inserted through the abdomen
- transcervical CVS – a tube is inserted through the cervix (the neck of the womb)
Read more about these methods and how CVS is performed.
The test takes about 5 minutes, although the whole consultation will take about 30 minutes. CVS has been described as uncomfortable rather than painful, and there may be some cramps afterwards which are similar to menstrual cramps.
The first result should be available within a few days, and this will tell you whether a major chromosome problem has been discovered.
The full results, including for smaller, rarer conditions, can take two to three weeks to come back. If the test is looking for a specific disorder, the results may take up to a month
How does CVS work?
At an early stage of pregnancy, the embryo divides into two parts:
- one part develops into the baby
- the other part develops into the placenta
The part of the embryo that forms the placenta starts out as finger-like sections that are called chorionic villi. These burrow into the wall of the womb to get close to the mother's blood vessels.
The chorionic villi are formed by the division of the fertilised egg, which means they have exactly the same DNA as the embryo, including any possible genetic abnormality. Any defect in the chorionic villi will also be present in the foetus.
