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Section 9 – What is chorionic villus sampling (CVS)?


CVS is a procedure during which an obstetrician removes a small amount of tissue from your placenta (afterbirth) during pregnancy. The cells in this tissue are tested in the laboratory to look at your baby’s chromosomes. You can usually have CVS between 11 and 14 weeks of pregnancy.

Having a CVS carries an additional risk of miscarriage which is likely to be below 0.5% (around 1 in 200) of pregnancies. The additional risk of miscarriage following CVS in a twin pregnancy is around 1%  (1 in 100) of pregnancies.

A miscarriage is most likely to happen up to three weeks after CVS. No one knows why this happens or who it will happen to. It can happen whether or not your baby has a chromosomal change. CVS is done early in pregnancy, which is when miscarriages are slightly more common in all pregnant women.

Because CVS is a specialised procedure, you may not be able to have it done locally. Instead, you may be offered an appointment at a different maternity unit.

Your midwife or obstetrician can explain the test to you. It is your choice if you want this procedure or not.

If you decide to go ahead with CVS, we will ask you to sign a consent form agreeing to the procedure before it is carried out.

Preparing for CVS

  • You can have breakfast or a light lunch before your appointment.
  • You will need a full bladder for the scan you will have before CVS.
  • You may be asked to empty your bladder for the CVS procedure itself.
  • You may be more comfortable if you wear loose clothing.
  • You can bring your partner or a friend with you for support during and after the procedure, but please don’t bring any children with you.
  • If possible, you should arrange for someone to drive you home.


Having CVS

The procedure takes about 20 minutes and you will have it done as an outpatient, usually in the antenatal clinic. You will be awake for the procedure, and lying down.

You will have an ultrasound scan before CVS. This is to check the baby’s position and to find the best place to take the sample from your placenta

The obstetrician will give you an injection of local anaesthetic to numb the skin of your abdomen. Your abdomen is then cleaned with an antiseptic solution to reduce the risk of infection.  The obstetrician inserts a needle through your skin and the wall of your womb, and then takes a small sample from the placenta. The obstetrician will be watching the ultrasound scan to guide the needle and so avoid getting close to your baby. You may find the test a little uncomfortable.

You will also be asked to give a blood sample so that the laboratory can be sure that the results they get from the CVS sample are for your baby rather than for you. Occasionally the procedure cannot be done due to the position of the baby. If this happens, the obstetrician may suggest that the procedure is done on another day.


What happens after the procedure?

After the procedure you should rest in the clinic for up to 30 minutes. You may have tummy cramps afterwards, rather like period pains.

Some obstetricians may advise you to take things easy for a couple of days after the procedure, and to avoid having sex, or doing any heavy lifting or strenuous exercise. You won’t need complete bed rest.

If you have any pain or discomfort, you can take a normal dose of paracetamol.

Most women are back to normal after two days.


What you should look out for

  • Severe pain, which you can’t control by taking mild painkillers (like paracetamol).
  • Any bleeding or unpleasant discharge from your vagina.
  • Any fluid leaking from your vagina.
  • If you suddenly feel unwell, with a high temperature or flu-like symptoms.

These symptoms do not always mean there is a problem, but you may need further care and attention. For advice, please contact:

  • the clinic where you had CVS, or
  • your midwife

If your blood group is D negative, you will be offered a test called cell free fetal DNA which will predict your baby’s blood group.

An injection of anti-D after the procedure will be recommended if you have not had the cell free fetal DNA test, or if your baby is predicted to be D positive or you have had an inconclusive result. This is to reduce the chance of antibodies developing in your blood, which could happen if your baby’s blood group is D positive.