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.
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.
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.
Media last reviewed: 21/10/2014
Next review due: 21/10/2016
The chorionic villi
At an early stage of pregnancy, the embryo divides into two parts. One part develops into the baby and the other part develops into the placenta.
The part of the embryo that forms the placenta starts out as finger-like sections called chorionic villi. These burrow into the wall of the womb to get close to the mother's blood vessels.
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Why chorionic villus sampling is used
Chorionic villus sampling (CVS) allows specific genetic or inherited disorders to be diagnosed in the foetus at a very early stage.
CVS is not routinely offered during pregnancy, it is only used if the results of previous tests or your medical history suggest you have an increased chance of having a baby with a genetic disorder.
Some of the conditions that can be detected by CVS include:
chromosomal conditions such as Down's syndrome, a disorder that typically causes some level of learning disability and a characteristic range of physical features, or Edward's syndrome, a disorder that can result in a miscarriage, stillbirth or developmental disability
genetic disorders such as cystic fibrosis, which makes bodily secretions thick and sticky, hindering the function of certain organs
disorders of the musculoskeletal system such as Duchenne muscular dystrophy, a genetic disorder that causes progressive muscle weakness and disability
blood disorders such as thalassaemia, a condition that affects your body’s ability to create red blood cells, or sickle-cell anaemia, which affects how your red blood cells carry oxygen around your body
metabolic disorders such as antitrypsin deficiency, where your body cannot produce the protein alpha-1 antitrypsin, or phenylketonuria, where your body cannot produce the enzyme phenylalanine hydroxylase
mental health conditions such as fragile X syndrome, which is a condition that can affect your appearance, intelligence and behaviour
As well as the above conditions, some other, less common conditions may also be diagnosed using CVS.
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.
If there is a possibility that your unborn baby may have a condition that CVS could detect, a specialist involved in carrying out the test will discuss this with you. You will be given advice about the risks of the procedure and information to help you cope with the results.
In some cases, you may be referred to a genetic counsellor, a healthcare professional who is trained in genetics, for further discussion and advice.
If a condition is identified that cannot be treated, or if it causes severe disability in the child, the child’s parents may decide to have a termination (abortion). If the parents choose to continue with the pregnancy, CVS provides early notice of the condition, giving them plenty of time to prepare for the challenges ahead.
What happens during chorionic villus sampling
Chorionic villus sampling (CVS) involves taking a sample of cells from the tissue of the placenta of a pregnant woman. These cells are called chorionic villi.
The procedure is always carried out under the continuous guidance of an ultrasound scan to ensure that nothing enters the amniotic sac (the protective sac that cushions the baby with fluid as it grows) or touches the baby.
CVS can be carried out using two different methods:
In most cases, the transabdominal technique is used.
During transabdominal CVS, your tummy is cleaned with antiseptic, you are given a local anaesthetic injection and a needle is inserted through your skin into the womb. The needle is guided towards 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 chorionic villi. After the sample is removed, the needle is taken out.
During transcervical CVS, a sample of the chorionic villi are collected through your cervix (the lower part or neck of the womb).
First, your vagina and cervix will be cleaned with antiseptic. A thin tube or forceps (smooth metal instruments that look like large tongs) are then inserted through your vagina and cervix, and are guided toward the placenta using the ultrasound scan.
Which method will be used?
The transabdominal method is preferred in most cases. This is because it can be harder to obtain a sample using the transcervical method, and the tube may need to be inserted more than once.
Transcervical CVS is also more likely to cause vaginal bleeding immediately after the procedure, which occurs in about one 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 is 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, but you may have a sore tummy afterwards.
Transcervical CVS feels similar to having a cervical screening test.
How long does it take?
The procedure itself usually takes around 10 minutes to perform. Afterwards, you will be monitored for anything up to an hour in case the test causes any reactions, such as heavy bleeding. You can then go home to rest.
After the procedure, it is normal to have cramps that are similar to menstrual cramps and light vaginal bleeding called 'spotting' for a day or two. You can take over-the-counter painkillers, such as paracetamol (but not NSAIDs such as ibuprofen or aspirin), if you experience any pain.
You may wish to avoid any strenuous activity for a couple of days.
Contact your midwife or the hospital where the procedure was carried out for advice as soon as possible if you develop the following symptoms after the procedure:
a high temperature of 38°C (100.4°F) or more
chills or shivering
heavy vaginal bleeding or discharge
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 to come back. If the test is looking for a specific disorder, the results can take up to a month.
Results of chorionic villus sampling
After chorionic villus sampling (CVS) has been carried out, the sample of chorionic villi will be taken to a laboratory so that it can be tested.
The number of chromosomes in the cells can be counted, and the structure of the chromosomes can be checked for any abnormalities. If the CVS is being carried out to test for a specific genetic disorder, the cells in the sample can also be tested for this.
CVS is estimated to give a conclusive result in more than 99% of cases. However, it cannot test for every birth defect, and it may not give conclusive results. In about 1% of cases, the results of CVS cannot establish with certainty that the chromosomes in the foetus are normal. Occasionally, the abnormality may be confined to the placenta and not the baby.
If this happens, it may be necessary to have further tests, such as examining the chromosomes in cells from the parents. It may also be necessary to have amniocentesis (an alternative test in which a sample of amniotic fluid is taken from the mother) to confirm a diagnosis.
Getting the results
The first result should be available within two to three working 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 to come back, although if the test is looking for a specific disorder the results may take up to a month.
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 will also receive a written confirmation of the results.
What the results mean
For most women who have CVS, the results of the procedure will be 'normal'. This means the baby will not have any of the disorders that were tested for.
However, it is sometimes possible to have a normal result but a baby that is born either with the condition that was tested for or with another genetic condition. This is because a normal test result cannot exclude every possible genetic disorder.
If your test is 'positive', your baby has one of the disorders it was tested for. In these cases, the implications will be fully discussed with you. There is no cure for most chromosomal conditions, so you will need to consider your options carefully. These include:
continuing with your pregnancy, while gathering information about the condition so that you are prepared for caring for your baby
having a termination (abortion)
If you are considering a termination, talk to your midwife or GP. They can give you important information and advice.
For example, your options for termination will depend on how many weeks pregnant you are when you make the decision. If you decide to end your pregnancy, you may wish to talk to a counsellor afterwards. Again, your GP or midwife can help you arrange this.
The organisation Antenatal Results and Choices also helps parents with any issues around antenatal testing and its implications.
Complications of chorionic villus sampling
Before you decide to have chorionic villus sampling (CVS), your GP, midwife or genetic counsellor will discuss the risks and possible complications with you.
CVS carries a risk of miscarriage, which is when the pregnancy is lost in the first 23 weeks.
The risk of miscarriage after CVS is estimated to be about 1-2%, although it is difficult to determine which miscarriages would have happened anyway and which are the result of the CVS procedure.
In most cases, miscarriages linked to CVS occur within two weeks of the procedure.
It is thought that the risk of miscarriage after CVS may be slightly higher than after an alternative test called amniocentesis.
In less than 1% of procedures, the sample of cells that is removed may not be suitable for testing. This could be because not enough cells were taken, or 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.
As with all types of surgical procedures, there is 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 one in every 1,000 procedures.
If your blood type is rhesus (RhD) negative but your baby’s blood type is RhD positive, it is possible for sensitisation to occur during CVS. Rhesus sensitisation is where some of your baby’s blood enters your bloodstream and your body starts to produce antibodies to attack it.
If it is not treated, this can cause rhesus disease in the baby. An injection of a medication called anti-D immunoglobulin is now widely used to prevent sensitisation occurring.