CVS

Chorionic Villus Sampling

What is it?

A chorionic villus sampling or CVS means sampling of the early placental tissue with a fine needle in order to get diagnostic information about the fetal chromosomes. This test is usually performed at 11 – 14 weeks of pregnancy in order to address concerns about the risk of chromosomal abnormality such as Down syndrome. Normal results from a CVS will confidently exclude all fetal chromosome abnormalities such as Down syndrome which is the most common known cause of major mental handicap.

Who should have a chorionic villus sampling?

The decision about whether or not to proceed with CVS rests solely with the prospective parents. Women at high risk for fetal chromosomal abnormality are the ones to most often consider CVS. These are for example older women who are 37 years or more, or those who have had high risk results from a first trimester ultrasound or combined biochemical screening test.

CVS can occasionally be used for testing of genetic syndromes such as cystic fibrosis however this application is specifically for those parents with a known family history risk. This usually requires prior genetic counseling and testing of the parents as potential carriers.

How is the test performed?

Prior to a CVS a scan is performed to check that the position of the developing placenta is suitable and that there is safe access for the sampling. A local anesthetic is administered with a fine needle prior to insertion of the sampling needle under ultrasound guidance. Care is taken to avoid the pregnancy sac and the fetus thereby minimizing the risk of amniotic fluid leakage.

Is it safe?

There is a small extra risk of miscarriage generally quoted as 1 in 100. This figure is low considering the background risk of miscarriage that exists for every pregnancy. CVS is therefore considered a safe test and there is no significant risk to the mother from the procedure. As mentioned above this highly specialised procedure is best performed by experts in this field and Dr. Shawn Choong at Northern Ultrasound for Women has extensive experience in prenatal fetal procedures including routine diagnostic testing with CVS.

How to best prepare for a CVS?

Please bring evidence of your blood group. No special preparation is necessary beyond that for a basic ultrasound scan which means attending with a comfortably full bladder. The following is also usually advisable:

• Bring someone with you who can drive you home afterwards.
• No heavy lifting for 24 hours after the procedure.

How long do the results take to come back?

The full results on all the detail of the fetal chromosomes is called a full banded karyotype and takes 12 – 14 days usually. This is because the cytogenetics laboratory require this time for the cells to divide and be examined microscopically. Your doctor will receive these results as soon as they are available. An additional test called FISH analysis can produce rapid results and is occasionally advised for extremely high risk cases particularly if the gestation is advanced. This is only 99% accurate and does not however cover the full detail of all the fetal chromosomes therefore the standard CVS testing as above is still carried out regardless of whether a FISH analysis has been requested.