About Chorionic Villus Sampling (CVS)
A CVS is undertaken to check the baby’s chromosomes for abnormalities and some specific genetic conditions. It involves using ultrasound to guide a fine needle into the placenta. A small sample of the placenta is then drawn off into a syringe and sent for testing.
CVS is usually considered if:
- if you are over 35 years of age (sometimes called advanced maternal age)
- if you have a high probability combined test or NIPT test result
- if you or your partner have a family history of a specific genetic condition
- if you or your partner have had a previous pregnancy affected by a specific genetic condition
- if a structural abnormality is identified on an ultrasound scan (for example, a cystic hygroma)
What are the risks of CVS?
The main risk is from miscarriage which occurs in about 1-2% of pregnancies following CVS. There is an increased risk of facial abnormalities and limb defects in pregnancies where early CVS has been performed before 10 weeks.
There is also a risk of abdominal cramps and vaginal bleeding but if mild this does not usually cause any problems and you can take paracetemol safely for the cramps. If there is heavy bleeding, severe pain, fever, vaginal discharge or vaginal fluid loss you should contact your midwife, hospital, GP or our centre.
Why do you need to know my blood group?
Women who are Rhesus Negative need an injection of anti-D within 72 hours to protect their baby against the development of antibodies in the mother which can cause anaemia and jaundice in the fetus.
When is CVS done and who will do the test?
CVS is performed after 10+3 weeks with us (as we are a specialist centre with fetal medicine expertise) and after 11 weeks in most NHS hospitals. All our CVS tests are carried out by Dr Bryan Beattie, Consultant in Fetal Medicine, who has performed over 500 CVS procedures in Cardiff.
What tests are done on the sample?
Three main chromosome tests applied to the fluid are a PCR, a conventional karyotype and an array CGH:
- The PCR works in 98% cases to give a result within 3 days for Down Syndrome (Trisomy 21), Edward Syndrome (Trisomy 18), Patau Syndrome (Trisomy 13) and if requested the sex of the baby.
- A karyotype takes about 14 days and will identify most of the rarer chromosomal problems not picked up by the PCR.
- Array CGH will identify up to 10% more chromosomal abnormalities that would not be picked up by a PCR and karyotype. This is the most accurate test and ideally we would advise this in all cases.
- We also test for the dF508 cystic fibrosis genes which are responsible for about 85% of cases of cystic fibrosis.
Sometimes the CVS may fail to give a result, give a result which is difficult to interpret or an unexpected result and if so it will be explained to you what implications this may have for the baby.
What do I do before and after the test?
The procedure requires no special preparation and takes about 20 minutes. Ideally try to arrange for someone else to drive you home. After resting for about 5-10 minutes you can go home to rest for at least 24 hours. You should book time off work the next day and try to arrange for help if you have small children to look after.
How will I get the results?
This will be discussed at the time of the CVS but will usually be by phone followed by a report posted to you, the hospital and your GP.