Chorionic villus sampling

Chorionic villus sampling (CVS) is a test that is carried out during pregnancy to detect serious problems with the foetus. 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 (inherited) defects.

CVS is available to women who are at greater risk of having a baby with an inherited disorder, such as Down's syndrome or muscular dystrophy. This could be because:

  • there is a family history of the condition
  • the pregnant woman is over 35 years of age

CVS is an alternative to amniocentesis (where a sample of the mother's amniotic fluid is taken for testing) and can be carried out earlier in the pregnancy. CVS is usually carried out between weeks 10 and 13 of the pregnancy, while amniocentesis cannot be carried out until week 15. 

What are chorionic villi? 

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 (uterus) 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 (genetic code) as the embryo, including any possible genetic abnormality. Any defect in the chorionic villi will also be present in the foetus.

The procedure

During CVS, a sample of chorionic villi cells will be taken from the pregnant woman's placenta using either:

  • transabdominal CVS - where a needle is inserted through the abdomen, or
  • transcervical CVS - a tube inserted through the cervix (the neck of the womb)

The test takes about 20 minutes and the results should be available within10-14 days.

CVS is not entirely without risk. In around two per cent of cases, the procedure can result in a miscarriage (the loss of the pregnancy).

Chorionic villus sampling
Chorionic villus sampling is a test during pregnancy to see if there are any genetic problems with the foetus. A small sample of tissue is taken from the placenta.

Genetic
Genetic is a term that refers to genes- the characteristics inherited from a family member.

Foetus
A foetus is an unborn baby, from the eighth week of pregnancy until birth.

Embryo
An embryo is an unborn baby, from when the female egg is fertilised by the sperm, until the eighth week of pregnancy.

Womb
The uterus (also known as the womb) is a hollow, pear-shaped organ in a woman where a baby grows during pregnancy.

Blood vessels
Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessels are veins, arteries and capillaries.

Amniocentesis
Amniocentesis is a medical test that is carried out during pregnancy in order to assess whether the unborn baby (foetus) could develop or has developed an abnormality or serious condition.

Amniotic fluid
The fluid that surrounds the foetus in the womb. The fluid contains cells that have been shed by the foetus.

Placenta
The organ that links a pregnant woman's blood supply to her unborn baby's.

Chorionic villus sampling (CVS) enables serious genetic disorders to be diagnosed at a very early stage.

If a condition is identified that cannot be treated, or if it causes severe disability in the child, 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 are listed below.

  • Chromosomal conditions, such as Down's syndrome, which is a genetic disorder that affects a person's physical appearance and their ability to develop mentally.
  • Genetic disorders, such as cystic fibrosis, which is a condition that makes your bodily secretions thick and sticky, hindering the function of certain organs.
  • Disorders of the musculoskeletal system - for example, Duchenne muscular dystrophy which is a genetic disorder that causes progressive muscle weakness and disability.
  • Blood disorders, such as thalassaemia which is a condition that affects your body's ability to create red blood cells, or sickle-cell anaemia, which is a condition that 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, there may be other, less common conditions that can also be diagnosed using CVS.

CVS cannot detect neural tube defects. These are birth defects that affect the brain and the spinal cord, such as spina bifida. Amniocentesis is an alternative test that takes a sample of the pregnant woman's amniotic fluid and can detect neural tube defects.

If there is a possibility that your unborn baby may have a condition that CVS could detect, your GP or midwife will discuss this with you. You will be given advice about the risks of the procedure and information to help you to cope with the results.

Genes
Genes are units of 'genetic material' that determine many of your body's characteristics, including the colour of your hair and eyes.

Chromosomes
Chromosomes are the parts of a cell in the body that carry genes. A human cell usually has 23 pairs of chromosomes.

Foetus
A foetus is an unborn baby from the eigth week of pregnancy until birth.

Amniocentesis
Amniocentesis is a medical test that is carried out during pregnancy in order to assess whether the unborn baby (foetus) could develop or has developed an abnormality or serious condition.

Amniotic fluid
The fluid that surrounds the foetus in the womb. The fluid contains cells that have been shed by the foetus.

Chorionic villus sampling (CVS) is carried out between weeks 10 and 13 of the pregnancy. It is not recommended earlier than this. The risk of CVS causing a miscarriage, or birth defects in the baby, is higher if it is carried out before week 10 of the pregnancy.

CVS is recommended 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 are over 35 years of age, as you are at increased risk of your child having Down's syndrome
  • 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)

CVS or amniocentesis?

Compared with amniocentesis, the advantage of CVS is that it can be performed earlier during the pregnancy. The results of CVS should take around 10-14 days. This allows time to repeat the procedure, or have amniocentesis if the results are inconclusive.

Amniocentesis, an alternative test for abnormalities, is usually carried out between week 15 and week 20 of the pregnancy. Results from amniocentesis can take two to three weeks to obtain. This means 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 GP or midwife will be able to discuss any appropriate tests with you, and explain why they might be necessary.

In some cases, you may be referred to a genetic counsellor (a healthcare professional who is trained in genetics). They will discuss your risk of passing on certain genetic conditions and will be able to offer you advice about what to do when you get the results.

Miscarriage
The loss of the pregnancy during the first 23 weeks.

Genes
Genes are units of 'genetic material' that determine many of your body's characteristics, including the colour of your hair and eyes.

Chromosomes
Chromosomes are the parts of a cell in the body that carry genes. A human cell usually has 23 pairs of chromosomes.

Foetus
A foetus is an unborn baby from the eigth week of pregnancy until birth.

Amniocentesis
Amniocentesis is a medical test that is carried out during pregnancy in order to assess whether the unborn baby (foetus) could develop or has developed an abnormality or serious condition.

Amniotic fluid
The fluid that surrounds the foetus in the womb. The fluid contains cells that have been shed by the foetus.

 

Chorionic villus sampling (CVS) involves obtaining a small sample of the chorionic villi (cells in the tissue of the placenta) from the pregnant woman. The procedure is always carried out under the guidance of an ultrasound scan to avoid making a hole in the amniotic sac (the protective sac that cushions the baby with fluid as it grows).

There are two different methods that can be used during the procedure:

  • transabdominal CVS
  • transcervical CVS

Transabdominal CVS

During transabdominal CVS, your abdomen is cleaned with antiseptic and a thin needle is inserted through your abdomen wall. The needle is guided towards your womb (uterus) using the image on the ultrasound scan.

Attached to the needle is a syringe (a device with a plunger and a hollow cylinder), which is used to extract a small sample of chorionic villi and then the needle is removed. 

Transcervical CVS

During transcervical CVS, the chorionic villi are collected by entering your body through your cervix (the lower part, or neck, of the womb). First of all, your vagina and cervix will be cleaned with antiseptic.

A thin tube is 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 and then the tube is removed.

Which method?

The choice of whether transabdominal CVS or transcervical CVS is used will depend on where your placenta is, because one method may be able to reach the chorionic villi better that the other.

Limited data suggests that transabdominal CVS may be slightly safer than transcervical CVS, but there is no difference in the rate of miscarriages between the two methods.

Transcervical CVS is considered more technically demanding. It can be harder to obtain a sample using this 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. Bleeding occurs in about 10 per cent of women who have this procedure.

Is CVS painful?

CVS has been described as being uncomfortable, rather than painful. Transcervical CVS is comparable to having a smear test (cervical screening). The cramps that some women experience afterwards are about as painful as menstrual cramps. Some women who have transabdominal CVS report having a sore abdomen after the procedure. 

One study of pain during CVS found that transabdominal CVS may be more painful for larger women because their abdominal wall is thicker. Transcervical CVS may be more painful for women who have not previously given birth. In some cases, a local anaesthetic (painkilling medication) can be used to numb the area.

How long does it take?

CVS usually takes around 20 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 will then be allowed to return home to rest.

After the procedure, you may experience cramps that are similar to menstrual cramps, and 'spotting' (drops of blood from your cervix). You may wish to avoid any strenuous activity for a day or two.

The results are usually available within 10 to 14 days.

Placenta
The organ that links the pregnant woman's blood supply to her unborn baby's.

Ultrasound
A procedure that uses high frequency sound waves to create an image of an organ in the body.

Womb
The womb (uterus) is the part of the female reproductive system where a fertilised egg develops into a baby during pregnancy.

Foetus
A foetus is an unborn baby from the eigth week of pregnancy until birth.

Miscarriage

Chorionic villus sampling (CVS) does carry a risk of miscarriage (when the pregnancy is lost in the fist 24 weeks). However, results can vary, with most estimates suggesting that CVS is slightly more dangerous than amniocentesis, which has a miscarriage risk of around one per cent.

One 16 year study found that the risk of miscarriage after CVS was 2.7 per cent, but in the control group of women, who did not have CVS, the risk of having a miscarriage was 3.3 per cent. This suggests that CVS does not significantly increase your risk of having a miscarriage. 

Another 11 year study found that the risk of miscarriage after having CVS was 1.9 per cent, which is the same result that was found by an earlier, large study . The Royal College of Obstetricians and Gynaecologists (RCOG) also advise that the risk of miscarriage following CVS is around two per cent.

One of the biggest influences on the risk of miscarriage in relation to CVS could be how many procedures a particular medical centre has carried out. For example, the 11 year study found that medical centres which performed more than 1,500 CVS procedures over this time had lower rates of miscarriage. This suggests that having increased experience of the procedure reduces the risk of miscarriage.

CVS failure

Sometimes, it is possible for the procedure to fail to extract a sample of chorionic villi that can be used for testing. This can happen in between 2.5-4.8 per cent 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.

Infection

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 that are being used. The risk of severe infection is thought to be very low, occurring in less than one in every 1,000 procedures that are carried out.

Recovery

Recovery from CVS should only take about a day. Some people may experience a small amount of bleeding, or cramps, after the procedure. You should contact your GP for advice if, after CVS, you have:

  • a high temperature (fever) of 38C (100.4F) or over
  • excess bleeding
  • excess vaginal discharge

Rhesus sensitisation

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 left untreated, this can cause Rhesus disease in the baby. An injection of a medication called anti-D immunoglobulin is now widely used in order to prevent sensitisation occurring.

Foetal abnormalities

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 in the pregnancy to avoid this possible complication.

Miscarriage
A miscarriage is the loss of the pregnancy during the first 23 weeks.

Amniocentesis
Amniocentesis is a medical test that is carried out during pregnancy in order to assess whether the unborn baby (foetus) could develop or has developed an abnormality or serious condition.

Amniotic fluid
The fluid that surrounds the foetus in the womb. The fluid contains cells that have been shed by the foetus.

Cervix
The opening, or neck of the womb.

Womb
The womb (uterus) is the part of the female reproductive system where a fertilised egg develops into a baby during pregnancy.

Foetus
A foetus is an unborn baby from the eigth week of pregnancy until birth.

After the chorionic villus sampling (CVS) procedure, 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 for a specific genetic disorder, the cells in the sample can also be tested for this.

CVS is estimated to be about 99 per cent accurate. However, it cannot test for every birth defect, and it may not give conclusive results. In about one per cent 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 testing, 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. 

Test results

For the majority of women who have CVS, the results of the procedure will be 'normal'. That is, your baby will not have any of the disorders that were tested for.

However, occasionally, it is possible to have a 'normal' result but the baby is born either with the condition that was tested for, or with another chromosomal (genetic) condition. This is because a 'normal' test result cannot exclude every possible genetic disorder.

If your test is 'positive' it means that 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. You should be aware that there is no cure for the majority of chromosomal conditions. Therefore, you need to consider your options carefully. These might include:

  • continuing with your pregnancy, while gathering information about the condition, so that you are prepared for caring for your baby
  • ending your pregnancy

If you are considering ending your pregnancy, you should talk to your GP, or midwife, who will be able to provide you with important information and advice.

For example, your options for ending your pregnancy 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 will be able to help you to arrange this.

Genes
Genes are units of 'genetic material' that determine many of your body's characteristics, including the colour of your hair and eyes.

Chromosomes
Chromosomes are the parts of a cell in the body that carry genes. A human cell usually has 23 pairs of chromosomes.

Counselling
Counselling is guided discussion with an independent trained person, to help you find your own answers to a problem or issue.

Termination
An abortion or termination is a medical procedure that ends pregnancy. The method used depends on the stage of pregnancy.

Foetus
A foetus is an unborn baby, from the eighth week of pregnancy until birth.

Amniocentesis
Amniocentesis is a medical test that is carried out during pregnancy in order to assess whether the unborn baby (foetus) could develop or has developed an abnormality or serious condition.

Amniotic fluid
The fluid that surrounds the foetus in the womb. The fluid contains cells that have been shed by the foetus.

 

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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