Before you decide to have chorionic villus sampling (CVS), your GP, midwife or genetic counsellor will discuss the risks and possible complications with you, which include miscarriage.

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%.

CVS failure

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.

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 being used. Severe infection occurs in less than 1 in every 1,000 procedures.

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 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.

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 of pregnancy to avoid this possible complication.