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Introduction 

Chorionic villus sampling (CVS) is a test you may be offered during pregnancy to check if your baby has a genetic disorder, such as Down’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).

When CVS is offered

CVS isn't routinely offered to all pregnant women. It's only offered if there's a high risk your baby could have a genetic condition.

This could be because:

It's important to remember that you don't have to have CVS if it's offered. It's up to you to decide whether you want it.

Your 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.

Why chorionic villus sampling is offered 

Chorionic villus sampling (CVS) is offered to pregnant women at risk of having a baby with a serious genetic disorder. It will diagnose any problems at an early stage.

CVS isn't routinely offered to all women during pregnancy. It's offered if your test results or medical or family history suggest you have an increased chance of having a baby with a genetic disorder.

You don't have to have the test if it's offered – it's up to you to decide whether you want it.

What conditions can CVS detect?

CVS can be used to diagnose a number of serious genetic disorders, including:

  • Down's syndrome – a condition that typically causes some level of learning disability and a characteristic range of physical features
  • Edward's syndrome and Patau's syndrome – conditions that can result in miscarriage, stillbirth or (in babies that survive) severe physical problems and learning disabilities
  • cystic fibrosis – a condition in which the lungs and digestive system become clogged with thick, sticky mucus
  • Duchenne muscular dystrophy – a condition that causes progressive muscle weakness and disability
  • thalassaemia – a condition that affects the red blood cells, which can cause anaemia, restricted growth and organ damage
  • sickle-cell anaemia – where the red blood cells develop abnormally and are unable to carry oxygen around the body properly
  • phenylketonuria – where your body cannot break down a substance called phenylalanine, which can build up to dangerous levels in the brain

CVS cannot detect neural tube defects. These are birth defects affecting the brain and the spinal cord, such as spina bifida, which can usually be detected with an ultrasound scan.

Deciding whether to have CVS

If you're offered CVS, ask your doctor or midwife what the procedure involves and what the risks and benefits are before deciding whether to have it.

You may also find it helpful to contact a support group, such as Antenatal Results and Choices (ARC). ARC is a charity that offers information, advice and support on all issues related to screening during pregnancy.

Reasons to have CVS

The test will usually tell you whether your baby will be born with any of the genetic conditions that were tested for.

If no problem is found, it may be reassuring. A result showing that a genetic condition was detected will give you plenty of time to decide how you want to proceed with your pregnancy. Read about the results of CVS for more information.

Reasons not to have CVS

There is a 1-2% chance you could have a miscarriage after the procedure. You may feel this risk outweighs the potential benefits of the test. Read more about the risks of CVS.

Some women decide they don't want to know if there's a problem with their baby until later on. You may choose to have an alternative test called amniocentesis later in your pregnancy instead, or you might just want to find out when your baby is born.

How CVS is performed

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 will be removed from the placenta using one of two methods:

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

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 day or two afterwards.

Read more about what happens during CVS.

Getting your results

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

If rarer conditions are also being tested for, it can take two to three weeks or more for the results to come back.

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

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

Read more about the results of CVS.

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 women who have CVS.

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

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.

Read more about the risks of CVS.

What are the alternatives?

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 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, so you'll have a bit less time to consider your options.

If you're 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 make a decision.


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