Chorionic villus sampling (CVS)

About CVS

This test is not offered as a matter of routine. It is used to detect Down’s, Edwards’ or Patau’s syndrome and other specific known genetic disorders. There is no evidence that the procedure itself harms the baby as the test is carried out under ultrasound guidance, but the test is most safely performed after 11+4 weeks of pregnancy. The procedure is done by an obstetric doctor trained to undertake CVS.

Who should consider CVS?

• If you have a received a high-chance result from NIPT, combined screening, or the quadruple test in this pregnancy.
• If there is a potential problem found on an ultrasound scan in this pregnancy, which may suggest a chromosomal abnormality.
• If you had a previous pregnancy or baby/child affected by a genetic condition that can be tested for by an amniocentesis, including Down’s, Edwards’ or Patau’s syndrome.
• If either biological parent has a family history of genetic disorders, including some rare inherited diseases that can be tested for by amniocentesis.
• If you who have missed the screening tests for Down’s, Edwards’ or Patau’s syndrome, or request invasive testing for your own reasons.

The age-related chance of having a baby with Down’s syndrome is as follows:

• 25: 1 in 1,500
• 30: 1 in 910
• 35: 1 in 380
• 38: 1 in 190
• 40: 1 in 110
• 45: 1 in 30

Benefits

It is the only way to know whether a baby’s chromosomes are affected by certain conditions.

Risks

CVS has now been available for a number of years. 7,000 to 8,000 tests are performed every year in Britain. We therefore know a lot about its safety and accuracy.

• We know that the test can sometimes cause a miscarriage for approximately one in 200 women (0.5%) who have the test.
• There is no evidence that the procedure itself harms your baby, as the test is carried out under ultrasound guidance.

Alternatives

Amniocentesis is available after 15 weeks of pregnancy.

Is the CVS test reliable?

No test is perfect, but the chromosome test for Down’s, Edwards’ and Patau’s syndromes is very reliable. It fails to give a clear result in less than 1% of cases.

If you are having the CVS test for a genetic disorder, you should discuss the accuracy of the test with your genetic doctor or counsellor.

Will the test reveal anything other than Down’s, Edwards’ or Patau’s syndrome?

You may be having this test to detect one of these conditions or another specific, known genetic disorder. Occasionally the test may detect problems with the other chromosomes. If the results show anything abnormal you will be told what the abnormality is and how this may affect your baby.

How is the CVS performed?

The test involves taking a sample of the developing placenta (afterbirth) that contains the chorionic tissue. Before the test is performed, an ultrasound scan is carried out to check your dates and the position of both the baby and the placenta (afterbirth).

The skin over the abdomen is cleaned with antiseptic and a local anaesthetic injection is given to numb the area. A fine needle is then passed through the wall of the womb into the chorionic tissue. Ultrasound is used to help the doctor to guide it into the right place. A small piece of the chorionic tissue (about the size of a few grains of rice) is removed through the needle and sent to the laboratory for testing.

Local anaesthesia

In local anaesthesia the local anaesthetic drug is injected into the skin and tissues at the site of the procedure. The area of numbness will be restricted and some sensation of pressure may be present, but there should be no pain. Usually, a local anaesthetic will be given by the doctor doing the procedure.

Is the CVS test painful?

Although you will be aware of what is happening, most women and birthing people describe it as uncomfortable rather than painful, similar to period pains. Most women and birthing people say they are aware of a ‘pushing’ feeling and some soreness over the area afterwards but that generally the thought of the procedure is worse than the actual test.

What happens after the CVS test?

You will probably be at the hospital for about half an hour, but the test itself only takes a few minutes.

If your blood group is Rhesus negative, you will be given an injection of anti-D following the procedure to prevent the formation of antibodies in your blood, which may otherwise affect future pregnancies.

We encourage you to bring a companion with you for support during and after the test. It is a good idea to take things easy for a couple of days, although this will not affect your risk of miscarriage.

It is not unusual to have some spotting for a few hours after the test. A 'period pain' feeling may last for 24 to 48 hours, and should settle after rest and paracetamol, which is safe to take in pregnancy. If you have excessive pain, are leaking any waters, experience vaginal bleeding or develop a high temperature, please contact 01223 217217 (maternity assessment) for advice; this line is answered by a midwife 24 hours a day, seven days a week. Alternatively, call your local maternity unit if you are not planning to have your baby at the Rosie Hospital.

About the CVS test results

When do I get the results?

The laboratory test involves the extraction of DNA from CVS cells. The DNA is tested using a rapid method called a QF-PCR test (quantitative fluorescent polymerase chain reaction). These tests will assess three of the possible 23 pairs of chromosomes in the baby.

The chromosomes that will be assessed will be chromosome 21, 18 and 13, as too many of these chromosomes in an individual are the most common cause of foetal abnormality in the population; namely, trisomy 21 (Down’s syndrome), trisomy 18 (Edwards' syndrome) and trisomy 13 (Patau’s syndrome).

These tests do not detect other chromosomal rearrangements (a structural change in a chromosome) or abnormalities of the sex chromosomes. If clinically indicated, sex chromosome tests and microarray may be undertaken (the microarray test can detect changes that cause extra or missing copies of genes, and a separate information leaflet and consent form are required).

The QF-PCR test usually takes about three working days. In certain situations a further test will check all chromosomes of the baby and this result will take about two weeks. If the chromosomes are normal, we will send you a written report by post.

If you are having an invasive test because of ultrasound anomaly, genetic conditions or history then the rapid QF-PCR and microarray will be performed. The rapid test result will be available within three working days and the microarray result will be available after 10 to 14 working days.

If you are having an invasive test due to an increased chance from Down’s/ Edwards’/ Patau’s screening then only the rapid QF-PCR test would be performed. If you wish to have the microarray test or a full karyotype then an additional charge would be made to cover the laboratory expenses involved in this.

As the full karyotype is not performed, it is anticipated that a small number of babies will be affected by clinically important chromosomal abnormalities which will not be detected by QF-PCR.

What if the results are abnormal?

If a chromosome problem is detected, you will usually be contacted by either the Fetal Medicine Unit or your own referring hospital. You will usually be contacted by telephone and given an opportunity to discuss the findings. You will be told what the abnormality is and how this could affect your baby. You will have the chance to discuss the results fully before making any decisions.

You will be contacted by one of:

• a genetics counsellor by prior arrangement
• the Fetal Medicine Unit at the Rosie Hospital, or
• your own referring hospital.

If you are having a CVS test done for a less common genetic problem, you need to talk to your genetic doctor or counsellor about how long the results will take, as this varies depending upon the information required and how you wish to be informed of the result.

HIV infection

We would encourage women who are HIV positive to discuss invasive testing with their specialist midwife or doctor to obtain further information. There is a small risk with CVS that the HIV virus can be passed on to the developing baby during the procedure. In these circumstances, it may be possible for women to have an alternative test called an amniocentesis. An information leaflet on the amniocentesis test is also available.

Outcome of pregnancy

Outcome information is very important to us as this enables us to audit and improve our service. Following your appointment with us, you will be given an outcome form and an addressed envelope, which we would be very grateful if you would complete and return to us.

Medication

Bring all your medicines (including inhalers, injections, creams, eye drops or patches), as well as a current repeat prescription from your GP. Please tell the ward staff about all the medicines you use and any allergies you have.

MyChart

We would encourage you to sign up for MyChart. This is the electronic patient portal at Cambridge University Hospitals that enables patients to securely access parts of their health record held within the hospital’s electronic patient record system (Epic). It is available via your home computer or mobile device.

Further information

If you have any questions about CVS, please telephone the Fetal Medicine Unit at the Rosie on 01223 216185. We can arrange for a Fetal Medicine midwife to talk to you if you wish. We recognise that everyone will have their own questions and concerns to discuss. The midwife or doctor will usually be able to answer any questions you may have before you have your test. Please also call us if you need to change or cancel your appointment for any reason (including illness) so your slot can be used by others.

Resources and references

• Antenatal Choices & Results (opens in a new tab) (ARC) charity – Helpline: 020 7713 7486
• Royal College of Obstetricians and Gynaecologists (opens in a new tab) (2021) Amniocentesis and chorionic villus sampling, green-top guideline no.8