A CVS is a diagnostic test performed between 11-14 weeks to assess the chromosomes of the fetus. It involves taking a sample of placental tissue (chorionic villi) with a needle passed through the mother’s abdominal wall under ultrasound vision.
Who may be offered a CVS?
- Women who have had a screening test suggesting the fetus has an increased risk of chromosomal abnormality
- Women who have previously had a child with chromosomal abnormality
- Women who because of family history are known to be at increased risk of an inherited genetic disease (eg cystic fibrosis or thalassaemia)
- Women who may be low risk for abnormality but want to definitively exclude chromosomal abnormality in their baby.
Why perform a CVS rather than an Amniocentesis?
CVS is performed in preference to an amniocentesis if the gestation is less than 15 weeks or (rarely) if there is insufficient fluid to be able to take an amniotic fluid sample after this time.
How is CVS performed?
An ultrasound is performed to check the position of the developing placenta and to confirm that the procedure is appropriate.
The mother’s abdomen is then cleaned with antiseptic
Local anaesthetic is injected to numb the skin
The CVS needle is then inserted under ultrasound guidance into the placenta through the mother’s abdominal wall and the sample is taken through this.
What are the side effects?
Most people report the experience of the CVS was better than they anticipated and was uncomfortable rather than painful
The local anaesthetic stings the skin while it is injected but the area then rapidly becomes numb
Period -type pain can be experienced as the needle is passed through the uterine wall and into the placenta and while the sample is obtained.
A “pinch” sensation may be felt as the needle is withdrawn.
A sensation of slight bruising or tenderness may continue for a day or so after the procedure.
What are the risks?
CVS is regarded as a safe procedure but does add a small increased risk of miscarriage to a patient’s pre-existing background risk. This is in the order of 1%.
Occasionally bleeding can occur after the procedure. If this is more than spotting, please contact us or your obstetrician.
Techinical failure at the time of testing or in the laboratory may occur and repeat testing or amniocentesis at 16 weeks may be necessary.
In 1% of cases the result may suggest “placental mosaicism”. This is a situation where a chromosomal abnormality is identified but it is thought to be confined to the placenta. An amniocentesis may be offered to clarify the result.
If a comprehensive chromosome test called a “microarray” is performed on the placental tissue, there is a small risk of a finding of “uncertain significance”. Parental blood tests are usually then required to clarify the result and counseling by a geneticist would be arranged to determine if the finding is significant in the individual case.
What happens following the test?
Patient’s are able to drive themselves home following the test but may prefer to have a companion do this. It is recommended that heavy lifting, exercise and sexual intercourse be avoided for 24 hours following the procedure.
The tissue sample is sent to a specialized genetics laboratory where the placental tissue is grown in an incubator for 10-14 days before the cells can be analysed.
A limited result (known as the “FISH” test) is available 24-48 hours after the procedure at extra cost. This looks for an extra 13th, 18th and 21st chromosome and for the sex chromosomes.
The full result of all the fetal chromosomes is known as a “karyotype” and is available 10-14 days after the procedure. Traditionally this has been a “banded karyotype” but new technology has allowed greater detail to be examined and this is known as a “microarray karyotype”. This has been shown to identify approximately 5% more chromosomal abnormalities than the banded karyotype but at the risk of 1-2% of a finding of uncertain significance.
The doctor performing the procedure will discuss the most appropriate test with you.
Additional information is available on The Victorian Clinical Genetic Services (VCGS) website here