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.

Read more about why CVS is offered and deciding whether to have it.

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.

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.

What happens during chorionic villus sampling

Chorionic villus sampling (CVS) involves taking a sample of cells from the tissue of the placenta (the ‘chorionic villi’).

Preparing for CVS

You won’t usually need to do anything special to prepare for CVS. You can eat and drink as normal beforehand.

In some cases, you may be advised to avoid going to the toilet for a few hours before the test, because it’s sometimes easier to perform when your bladder is full. Your doctor or midwife will tell you about this before you attend your appointment.

You may want to bring a partner, friend or family member for support when you have the test.

How CVS is performed

CVS is carried out under the continuous guidance of an ultrasound scan. This is to make sure nothing enters the amniotic sac (protective sac that cushions the baby) or touches the baby.

The test can be carried out using two different methods: transabdominal CVS and transcervical CVS.

Transabdominal CVS

Your tummy is first cleaned with antiseptic before a local anaesthetic injection is used to numb it.

A needle is then inserted through your skin into the womb. The needle is guided to the placenta using the image on the ultrasound scan.

Attached to the needle is a syringe, which is used to extract a small sample of cells from the chorionic villi. After the sample is removed, the needle is removed. 

Transcervical CVS

A sample of cells from the chorionic villi is collected through your cervix (the neck of the womb).

A thin tube attached to a syringe or small forceps are inserted through your vagina and cervix, and guided towards the placenta using the ultrasound scan.

Which method will be used?

The transabdominal method is preferred in most cases, because it’s often easier to carry out.

Transcervical CVS is also more likely to cause vaginal bleeding immediately after the procedure, which occurs in about 1 in every 10 women who have this procedure. There is, however, no difference in the rate of miscarriages between the two methods.

Transcervical CVS may be preferred to transabdominal CVS if it’s easier to reach your placenta this way.

Is CVS painful?

CVS is usually described as being uncomfortable, rather than painful.

In most cases, an injection of local anaesthetic will be given before transabdominal CVS to numb the area where the needle is inserted, although you may have a sore tummy afterwards.

Transcervical CVS feels similar to a cervical screening test.

How long does it take?

The procedure usually takes around 10 minutes to perform.

Afterwards, you will be monitored for up to an hour, in case the test causes any side effects, such as heavy bleeding. You can then go home to rest.

It’s a good idea to arrange for someone to drive you home, as you might not feel up to it yourself.

Recovering after CVS

After CVS, it’s normal to have cramps similar to period pain and light vaginal bleeding called “spotting” for a day or two. You can take over-the-counter painkillers such as paracetamol (but not ibuprofen or aspirin) if you experience any discomfort.

You may wish to avoid any strenuous activity for the rest of the day.

Contact your midwife or the hospital where the procedure was carried out for advice as soon as possible if you develop any of the following symptoms after the procedure:

  • persistent or severe pain
  • a high temperature of 38C (100.4F) or more
  • chills or shivering
  • heavy vaginal bleeding
  • discharge of clear fluid from the vagina
  • contractions

Getting the results

The first results should be available within a few days, and this will tell you whether a major chromosome problem 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.

You can usually choose whether to get the results over the phone or during a face-to-face meeting at the hospital or at home.

Read more about the results of CVS.

Results of chorionic villus sampling

After chorionic villus sampling (CVS) has been carried out, the sample of cells will be sent to a laboratory to be tested.

The number of chromosomes (bundles of genes) in the cells can be counted, and the structure of the chromosomes can be checked for any abnormalities.

If CVS is being carried out to test for a specific genetic disorder, the cells in the sample can also be tested for this.

Getting the results

The first results should be available within three working days, and this will tell you whether a major chromosome problem such as Down’s syndrome has been found.

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

You can usually choose whether to get the results over the phone or during a face-to-face meeting at the hospital or at home. You’ll also receive written confirmation of the results.

How reliable are the results?

CVS is estimated to give a definitive result in around 99% of cases. However, it cannot test for every birth defect and it’s not always possible to get a conclusive result.

In a very small number of cases, the results of CVS cannot establish with certainty that the chromosomes in the baby are normal or not. This might be because the sample of cells removed was too small or there’s a possibility the abnormality is just in the placenta and not the baby.

If this happens, it may be necessary to have amniocentesis (an alternative test, in which a sample of amniotic fluid is taken from the mother) a few weeks later to confirm a diagnosis.

What the results mean

For many women who have CVS, the results of the procedure will be “normal”. This means that none of the genetic disorders that were tested for were found in the baby.

However, a normal result doesn’t guarantee that your baby will be completely healthy, as the test only checks for conditions caused by faulty genes, and it cannot exclude every possible genetic disorder.

If your test is “positive”, your baby has one of the disorders it was tested for. In this instance, the implications will be fully discussed with you and you’ll need to decide how to proceed.

What happens if a problem is found

If the test finds that your baby will be born with a genetic disorder, you’ll speak to a number of specialists about what this means.

This may be your midwife, a doctor with experience in childhood genetic conditions, a geneticist and/or a genetic counsellor.

They’ll be able to give you detailed information about the condition – including the problems your child may have, the treatment and support they might need, and whether their life expectancy will be affected – to help you decide what to do.

There’s no cure for most genetic conditions, so you’ll need to consider your options carefully. Your main options are:

This can be a very difficult decision, but you don’t have to make it on your own.

As well as discussing it with specialist healthcare professionals, talk things over with your partner and speak to close friends and family, if you think it might help. The charity Antenatal Results and Choices (ARC) can also offer support and impartial information.

Risks of chorionic villus sampling

Before you decide to have chorionic villus sampling (CVS), you’ll be told about the risks and possible complications.

The main risks associated with the procedure are outlined below.


CVS carries a risk of miscarriage, which is the loss of a pregnancy within the first 23 weeks.

The risk of miscarriage after CVS is estimated to be about 1-2%. This means that 1 in every 50 to 100 women will have a miscarriage after having CVS.

However, it’s difficult to determine which miscarriages would have happened anyway, and which are the result of the CVS procedure. Some recent research has suggested that only a very small number of miscarriages that occur after CVS are a direct result of the procedure.

In most cases, miscarriages linked to CVS occur within two weeks of the procedure. There’s no evidence to suggest you can do anything during this time to reduce your risk.

The risk of miscarriage after CVS is considered to be slightly higher than after an alternative test called amniocentesis, which is carried out slightly later in pregnancy.

Inadequate sample

In around 1% of procedures, the sample of cells removed may not be suitable for testing. This could be because not enough cells were taken, or because the sample was contaminated with cells from the mother.

If the sample is unsuitable, it may be necessary for the CVS procedure to be carried out again, or to wait a few weeks to have amniocentesis instead.


As with all types of surgical procedures, there’s a risk of infection during or after CVS. Infection can occur if there are bacteria on your skin or on the instruments being used.

However, severe infection occurs in less than 1 in every 1,000 procedures.

Rhesus sensitisation

If your blood type is rhesus (RhD) negative, but your baby’s blood type is RhD positive, it’s possible for sensitisation to occur during CVS. 

This is where some of your baby’s blood enters your bloodstream and your body starts to produce antibodies to attack it. If it’s not treated, this can cause the baby to develop rhesus disease.

If you don’t already know your blood type, a blood test will be carried out before CVS to see if there’s a risk of sensitisation. An injection of a medication called anti-D immunoglobulin can be given to stop sensitisation occurring, if necessary.

Read more about preventing rhesus disease.