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:
- you have had a previous pregnancy with foetal problems, such as a baby born with a chromosome abnormality or a mental health condition
- you have a family history of a condition such as cystic fibrosis or muscular dystrophy
- an earlier antenatal screening test has suggested that there may be a problem, such as sickle cell anaemia (an inherited blood disorder)
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 doctor 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.
Before you decide to have chorionic villus sampling (CVS), your doctor, midwife or genetic counsellor will discuss the risks and possible complications with you, which include miscarriage.
CVS carries an additional risk of miscarriage, when the pregnancy is lost in the first 23 weeks. According to the Royal College of Obstetricians and Gynaecologists (RCOG), the additional risk of miscarriage after CVS is around 2%. Most estimates suggest that CVS is slightly more dangerous than amniocentesis, which itself has a miscarriage risk of around 1%.
Sometimes, the procedure may fail to extract a sample of chorionic villi that can be used for testing. It happens in up to 5% of procedures. This could be because not enough cells were taken, or the sample was contaminated with cells from the mother. Failure is more likely in transcervical CVS because the procedure is more technically difficult. In such cases, it may be necessary to repeat the procedure.
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. Severe infection occurs in less than 1 in every 1,000 procedures.
If your blood type is rhesus-negative but your baby’s blood type is rhesus-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.
There have been some cases of limb abnormalities in the foetus, such as missing or short fingers and toes. Some clinical trials have made an association between these abnormalities and having CVS earlier than 10 weeks into your pregnancy. While the association is not conclusive, CVS is no longer performed earlier than 10 weeks of pregnancy to avoid this possible complication.
After chorionic villus sampling (CVS) has been carried out, the sample of chorionic villi will be taken to a laboratory so that the cells can be examined under a microscope.
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 be about 99% accurate. 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.
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.
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 smaller, rarer conditions, 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.
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 occasionally 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 the disorder that was being tested for. If you receive a positive test result, the implications will be fully discussed with you. There is no cure for most chromosomal conditions. Therefore, you need to consider your options carefully. These will include:
- continuing with your pregnancy, while gathering information about the condition so that you are prepared for caring for your baby
- having an abortion
If you are considering an abortion, talk to your doctor or midwife. They can give you important information and advice.
For example, your options for abortion 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 doctor 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.
How is it performed
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 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:
- transabdominal CVS
- transcervical CVS
In most cases, the transabdominal technique is used.
During transabdominal CVS, your tummy is cleaned with antiseptic and a needle is inserted through it. The needle is guided towards your womb 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 tube is then inserted through your vagina and cervix, and is guided toward the chorionic villi using the ultrasound scan. Gentle suction is used to remove a small sample of the chorionic villi. The tube is then removed.
Which method will be used?
In most cases, the transabdominal method is preferred. 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 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 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 smear 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, if you experience any pain.
You may wish to avoid any strenuous activity for a couple of days.
Contact your midwife, doctor, 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.
The full results, including rarer conditions, 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.
Read more about the results of CVS.
Alfirevic Z et al. Amniocentesis and chorionic villus sampling for prenatal diagnosis. Cochrane Database of Systematic Reviews. 2008, Issue 2. Art. No.: CD003252. DOI: 10.1002/14651858.CD003252
Odibo AO et al. Evaluating the Rate and Risk Factors for Fetal Loss After Chorionic Villus Sampling. Obstetrics and Gynecology. October 2008. Vol./is. 112/4.
Tabor A et al. Fetal loss rate after chorionic villus sampling and amniocentesis: an 11-year national registry study. Ultrasound in Obstetrics and Gynecology. July 2009. Vol/is 34/1.
Why it should be done
Chorionic villus sampling (CVS) allows specific genetic disorders to be diagnosed in the foetus at a very early stage.
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 an 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.
Some of the foetal disorders that can be detected by CVS include:
- chromosomal conditions such as Down's syndrome, a disorder that affects a person’s physical appearance and their ability to develop mentally
- 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.
If there is a possibility that your unborn baby may have a condition that CVS could detect, your doctor or midwife 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.