Chorionic villus sampling

About chorionic villus sampling

CVS involves taking a small sample of tissue from the placenta in your womb (uterus) to check for genetic abnormalities in your baby.

The placenta is an organ attached to the lining of your uterus during pregnancy. It provides oxygen and nourishment to your growing baby through the umbilical cord and it gets rid of waste products from your baby. The placenta is usually genetically the same as your developing baby so it can provide information about your baby's health.

There are two ways of performing CVS: it’s usually carried out through your abdomen (tummy), but can also be done through your cervix (neck of the womb). The doctor carrying out the procedure will advise you which is the best method for you, depending on the position of your placenta. CVS is usually carried out between 11 and 14 weeks of pregnancy, although it can be done later if necessary.

Why have chorionic villus sampling?

CVS can detect certain chromosome disorders in your baby including Down's syndrome (trisomy 21) and Edward's syndrome (trisomy18).

Chromosomes are structures in every cell in your body that contain the genetic instructions for life (genes), inherited from your parents. Normally, our cells contain 46 chromosomes: 23 inherited from each parent. Most people who have Down's syndrome have an extra chromosome 21 and most people with Edward's syndrome have an extra chromosome 18.

CVS can also be used to detect some other genetic (inherited) disorders, such as sickle cell anaemia, cystic fibrosis or thalassaemia.

CVS can't be used to diagnose conditions that aren't related to your baby's genetic make-up, such as spina bifida. Some of these conditions need to be diagnosed using amniocentesis to get a sample of the amniotic fluid, or by ultrasound.

It’s your choice whether or not to have CVS. You will usually be offered the test if:

  • a screening test has suggested that there is a high risk of your baby having Down's syndrome
  • you have had a previous pregnancy affected by a genetic disorder
  • you or your partner have an inherited condition such as sickle cell anaemia, or there is a history of certain genetic disorders in your family

Deciding to have chorionic villus sampling

Before deciding to have CVS, it's worth thinking about what the results will mean to you.

If the results show that your baby has normal chromosomes and no other obvious problems, this is reassuring. However, CVS can't test for every disease or guarantee that your baby will be born completely healthy.

If the test results show there is a problem, you will need to make some decisions, including whether to continue with or terminate the pregnancy (abortion). These decisions may be difficult, and it's a good idea to discuss the issues with your partner, family, doctor, genetic counsellor or midwife before deciding to have CVS. Some women choose to have CVS even though they would not terminate the pregnancy, so that if a disorder is identified, it can allow them time to adjust and prepare for their baby’s birth.

Only you can make the final decision about whether to have CVS. You will need to weigh up how important it is to you to have early information about whether your baby has a genetic disorder, against the risks involved in having the procedure. It’s important to take your time before you decide and feel that you have been given a chance to ask any questions you may have.

Preparing for your chorionic villus sampling

Your doctor or midwife will explain how to prepare for your test. CVS is usually done as a day-case. This means you will have the test and go home on the same day.

You may be asked to have a full bladder when you come for your appointment, but your doctor will let you know before you come.

Your doctor will discuss with you what will happen before, during and after your procedure, and any pain you might have. This is your opportunity to understand what’s involved, and you can help yourself by preparing questions to ask about the risks, benefits and any alternatives to the procedure. This will help you to be informed, so you can give your consent for the procedure to go ahead, which you may be asked to do by signing a consent form.

About the procedure

There are two ways of performing CVS: through your abdomen (transabdominal) or through your cervix (transcervical).

Transabdominal chorionic villus sampling

Your doctor will use an ultrasound device to check the position of your baby and the placenta in your womb.

Watching this image, he or she will guide a long, fine needle through the wall of your abdomen and towards the placenta. This can be slightly uncomfortable, so you may have a local anaesthetic to numb the skin where the needle will be inserted. Your doctor will take a tiny amount of tissue from your placenta and remove the needle. Your doctor will then check your baby again with the ultrasound.

Transcervical chorionic villus sampling

Your doctor will use an ultrasound device to check the position of your baby and the placenta in your womb, and to continue monitoring your baby throughout the procedure. He or she will use a device called a speculum to open your vagina. Your doctor will clean your vagina and cervix, before guiding a fine tube (catheter) through your cervix to the placenta. He or she will use the suction from a syringe on the end of the catheter to take a sample of the placenta. The catheter will then be removed and your baby checked again with the ultrasound.

If you have a rhesus negative blood type, you will be offered an injection of anti-D after the procedure. This prevents your body reacting to your baby’s blood if his or her blood group is different. See our frequently asked questions for more information about having a rhesus negative blood group and what this means if you’re having CVS.

Getting your results

The sample of tissue from your placenta will be sent to a laboratory for testing. Certain conditions (for example, Down's syndrome and Edward's syndrome) can be identified within three days of having CVS. For other conditions it can take a laboratory two to three weeks to provide a final result.

You can usually choose whether to receive your results by phone, letter, or come into the hospital to receive them. Your midwife may also be able to come to your home to give you your results.

For most women, the test will give a very clear result as to whether or not the chromosomes are normal, or for other conditions, positive or negative. Occasionally, a different condition is detected than the one that was originally being checked for. Rarely your baby may be born with another genetic or chromosomal disorder despite you having a normal result.

If your test is unclear or gives an uncertain result, your doctor may recommend further testing, usually with an amniocentesis.

What are the risks?

CVS is commonly performed and generally safe. For many women, the benefits of a clear diagnosis of any problem with their baby are greater than the potential risks. However, other women decide that the risks outweigh the benefits. The final decision is yours, and in order to make an informed decision and give your consent, you need to be aware of the possible side-effects and the risk of complications of this procedure.


Side-effects are the unwanted but mostly temporary effects you may get after having the procedure. You may have some mild pain or cramps, like period pain, and spotting (light bleeding from your vagina) for a few hours afterwards.

If you need them, you can take over-the-counter painkillers such as paracetamol. Follow the instructions in the patient information leaflet that comes with the medicine and ask your pharmacist for advice.

If you have any other side-effects such as feeling shivery and unwell, fluid loss, heavy bleeding or contractions, contact your doctor or midwife, or the hospital where you had your test immediately.


This is when problems occur during or after CVS. Most women are not affected, but the risks of CVS include the following.

  • Injury to you or your baby from the needle – using ultrasound to guide the needle reduces this risk.
  • You may develop an infection if the needle has punctured your bowel or because of skin contamination, but this is very rare.
  • There is a slight risk that your blood will be exposed to your baby's blood. This is only an issue if your blood is rhesus-negative and your baby's is rhesus-positive. This risk is reduced when you're given an injection of the appropriate antibodies after your test.

About one to two women in every 100 who have CVS may have a miscarriage as a direct result of the procedure.

The exact risks are specific to you and will differ for every person. Ask your doctor to explain how these risks apply to you.

How can I know if chorionic villus sampling is right for me?


Your doctor will explain to you the risks and benefits of having chorionic villus sampling (CVS). Only you can decide whether the test is right for you.


Making a decision about whether or not to have a diagnostic test during pregnancy can be difficult. You may find it helpful to discuss the options with your partner or family.

To help you make the decision that is best for you, your doctor should discuss with you:

  • the types of tests available and what the results will tell you
  • how reliable the tests are
  • the risk is of having an uncertain result and needing a repeat test
  • your risk of miscarriage from having the test
  • the timing of the test during pregnancy
  • how long the results will take and how you will get them
  • your options if your baby is found to have a disorder

Your doctor will be able to tell you about the disorders that can be detected by CVS. You may also want to know more about what is involved in ending a pregnancy and how you may feel afterwards.

If you think you're likely to decide to end your pregnancy if the results are abnormal, you might want to find out sooner about any possible disorders. In this case, you may decide that CVS is the best option for you because you will get the results sooner than with amniocentesis.

Will chorionic villus sampling harm my baby?


There is a small risk that you may miscarry as a result of having CVS.


CVS may cause a miscarriage if the needle punctures the amniotic sac and injures your baby or introduces an infection into your womb. Up to two in 100 women who have CVS will miscarry as a result of the procedure.

The risk is dependent on the skill of the doctor as well as the technique. A very experienced doctor may have a higher success rate and a lower rate of procedure-related miscarriage than a doctor who only carries out the procedure occasionally.

Don't shy away from asking your doctor about his or her level of experience in this technique. The Royal College of Obstetricians and Gynaecologists recommends that a doctor should do at least 30 CVS tests every year.

If you have any concerns about CVS, speak to your doctor or midwife.

Can I have chorionic villus sampling if my blood group is rhesus-negative?


Yes, you can have CVS if you're rhesus (RhD)-negative. You will be offered an injection after the test, which helps to stop your immune system from reacting to your baby's blood, in case he or she is rhesus-positive.


As well as the blood groups A, B, AB and O, RhD is another way to categorise the type of blood you have. If your blood group is RhD-positive, this means you have RhD antigens on the surface of your red blood cells. RhD-negative means you don't have these antigens. An antigen is a type of protein that can cause your immune system to react. If your immune system doesn't recognise an antigen, it will produce antibodies to attack it. Antibodies are proteins made by your immune system that usually fight against viruses and bacteria. Your baby won’t necessarily have the same blood type as you.

There is a risk that during a procedure such as amniocentesis or CVS, some of your baby's blood may enter your bloodstream. This doesn’t usually cause any problems, but if your baby’s blood is RhD-positive and you are RhD-negative, your immune system may react to the RhD antigen on your baby's blood cells and make antibodies against it (anti-D antibodies). This process is called sensitisation or alloimmunisation and it may cause problems in future pregnancies. To prevent this, your doctor will recommend that you have an injection of an antibody called anti-D after CVS. The injection helps to stop your immune system from reacting to your baby's blood. It won't harm your baby. You may be offered further injections if necessary.

What are my options if chorionic villus sampling results show a genetic disorder?


Most of the time, there isn't a cure for genetic disorders but there may be treatment available. You will need to decide what is best for you and your baby.


CVS, like amniocentesis, usually gives a clear result as to whether the chromosomes are normal or not, or for other conditions, positive or not. Your doctor will explain what the results mean. For the majority of genetic disorders, there isn't a cure.

How you choose to proceed with your pregnancy will depend on what the results will mean for you, your family and your baby. You may choose to continue with your pregnancy and prepare yourself and your family for the birth of a baby with health problems. If the baby has a severe defect, you may decide to end the pregnancy (abortion).

These decisions can be difficult. You may wish to discuss the issues with your partner, family, doctor, genetic counsellor or your midwife before deciding to have CVS or amniocentesis.

What is the difference between chorionic villus sampling and amniocentesis?


Although CVS and amniocentesis are similar, there are some significant differences.


CVS involves taking a small sample of the placenta from your womb. This can be done through your cervix or through your abdomen (tummy). Amniocentesis involves taking a small sample of the amniotic fluid surrounding the baby in your womb. This is done through your abdomen. Both tests use ultrasound to check the position of your baby. This helps to find the best place to remove a sample without damaging the placenta, the umbilical cord or your baby.

CVS is usually carried out earlier in pregnancy – between 11 and 14 weeks of pregnancy, whereas amniocentesis is usually done after 15 weeks of pregnancy. This time difference may affect what is involved in ending your pregnancy if you choose to do so.

The risk of having a miscarriage is rare after both CVS and amniocentesis. However, the risk may be slightly higher after CVS (one to two in 100) than after amniocentesis (one in 100). The exact risks are specific to you and will differ for every person. Ask your doctor to explain how these risks apply to you.

Deciding whether to have amniocentesis or CVS may be difficult, and it’s a good idea to discuss the issues with your partner, family, doctor, genetic counsellor or your midwife before deciding to have either test.