Chorionic villus sampling (CVS) is a test you may be offered during pregnancy to check if your baby has a genetic or chromosomal condition, such as Down's syndrome, Edwards' syndrome or Patau's syndrome.
It involves removing and testing a small sample of cells from the placenta, the organ linking the mother's blood supply with her unborn baby's.
CVS is not routinely offered to all pregnant women.
It's only offered if there's a high risk your baby could have a genetic or chromosomal condition.
This could be because:
It's important to remember that you do not have to have CVS if it's offered. It's up to you to decide whether you want it.
A midwife or doctor will speak to you about what the test involves, and let you know what the possible benefits and risks are, to help you make a decision.
CVS is usually carried out between the 11th and 14th weeks of pregnancy, although it's sometimes performed later than this if necessary.
During the test, a small sample of cells is removed from the placenta using 1 of 2 methods:
The test itself takes about 10 minutes, although the whole consultation may take about 30 minutes.
The CVS procedure is usually described as being uncomfortable rather than painful, although you may experience some cramps that are similar to period pains for a few hours afterwards.
The first results of the test should be available within 3 working days and this will tell you if Down's syndrome, Edwards' syndrome or Patau's syndrome has been discovered.
If rarer conditions are also being tested for, it can take 2 to 3 weeks or more for the results to come back.
A specialist doctor (obstetrician) or midwife will explain what the screening results mean and talk to you about your options.
There's no cure for most of the conditions found by CVS, so you'll need to consider your options carefully.
You may decide to continue with your pregnancy while gathering information about the condition so you're fully prepared.
Or you may consider ending your pregnancy (having a termination).
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, the loss of the pregnancy in the first 23 weeks.
This is estimated to occur in up to 1 out of every 100 women who have CVS.
There are also some other risks, such as infection or needing to have the procedure again because it was not possible to accurately test the first sample.
The risk of CVS causing complications is higher if it's carried out before the 10th week of pregnancy, which is why the test is only carried out after this point.
An alternative to CVS is a test called amniocentesis.
This is where a small sample of amniotic fluid, the fluid that surrounds the baby in the womb, is removed for testing.
It's usually carried out between the 15th and 18th week of pregnancy, although it can be performed later than this if necessary.
This test has a similar risk of causing a miscarriage, but your pregnancy will be at a more advanced stage before you can get the results, so you'll have less time to consider your options.
If you're offered tests to look for a genetic or chromosomal condition in your baby, a specialist involved in carrying out the test will be able to discuss the different options with you and help you make a decision.