Chorion Villus Sampling (CVS)
What is a Chorion Villus Sampling?
CVS is a prenatal diagnostic test in which a small amount of placental tissue (known as chorionic villi) is obtained to enable assessment of fetal chromosomes or DNA.
The placenta usually has the same genetic structure as the fetus and can be accessed safely in the first trimester, a time when amniocentesis (in which amniotic fluid is sampled) is not able to be performed.
Why would a CVS be performed?
A CVS is performed to assess the fetal chromosomes in pregnancies at high risk for chromosome problems (eg. Increased risk at the screening First Trimester Screen, previous affected pregnancies, parental chromosome problem). At Women’s Imaging Services the vast majority of CVS procedures are performed abdominally (some units use a transvaginal approach). CVS is performed with the use of continuous ultrasound guidance to facilitate accurate placement of the biopsy needle into the placenta.
A preliminary ultrasound examination is conducted to assess the pregnancy and pelvic structures. Following the application of povidone iodine to sterilise the maternal skin, a 20 gauge needle is inserted through the abdominal wall, after the administration of local anaesthetic, into the placenta. The ultrasound transducer rests on the maternal skin at all times to ensure the needle enters only the placenta and does not pierce the amniotic sac (in which the fetus lies). A 10 cc syringe is attached to the CVS needle once it is correctly positioned within the placenta and a small amount of tissue is aspirated. The needle is within the placenta for about 30-60 seconds. In general an adequate amount of placental tissue is obtained with a single needle insertion, although on occasions a second needle insertion may be required.
Documentation of the maternal blood group is required at the time of the CVS. Women who are Rhesus negative will need to have an injection of anti-D following the CVS (which will be administered by the medical staff at Women’s Imaging Services immediately following the CVS).
When will the test results be available?
In general, a preliminary chromosome result is available in 24- 36 hours for high risk cases. The full fetal chromosome result will take 10-14 days on average.
Normal Male Karyotype
Normal Female Karyotype
These two pictures demonstrate a normal chromosome result for a boy (upper) and girl (lower). In a normal fetus there are 46 chromosomes in each cell of the body (22 pairs of autosomes and 2 sex determining chromosomes).
For situations where fetal DNA analysis is required (eg. cystic fibrosis testing) the specific test being requested will determine the time for the result (on average 3-5 days, but on occasions longer depending upon the specific molecular tests requested).
Are there any risks to having a CVS?
A CVS is an invasive diagnostic procedure and does carry with it a small but definite pregnancy loss rate (in the order of 1-2%). As CVS is performed earlier in pregnancy than amniocentesis, the quoted loss rate is higher, but this is more a feature of the increased background loss rate in the first trimester compared with the second trimester rather than any significant procedure-related effect. The presence of uterine fibroids (benign tumours of the uterine muscle) or prior vaginal bleeding may increase the pregnancy loss rate following a CVS.
It is common to feel some uterine cramps or discomfort following an abdominal CVS which settles within a few hours. Vaginal bleeding or infection are rare following an abdominal CVS.
We strongly suggest you have some-one attend your CVS appointment with you and have them drive you home. It is not appropriate to have your children attend for a CVS procedure and we would request you arrange childcare when scheduling for this procedure.
What are the advantages of a CVS?
The main advantage of a CVS test is the earlier gestation that it is performed at (typically 12 weeks) and hence the availability of results before the end of the first trimester of pregnancy. Although most CVS tests will be tests of reassurance, in situations where the results are not normal there is more flexibility and safety as regards management options for the woman.
When fetal DNA studies are needed, CVS is the procedure of choice as it enables the provision of large amounts of DNA for the molecular laboratory to facilitate a timely accurate result.
A preliminary chromosome result may be available in 24-36 hours in very high risk cases.
Are there any disadvantages to a CVS?
As discussed above, there is a pregnancy loss risk of 1-2% following a CVS, even in experienced hands. The pregnancy loss rate increases with the number of attempts required to obtain the chorionic tissue and we will not perform more than two attempts (although in the majority of cases we require only a single needle pass).
On a few occasions it may not be technically possible to safely perform a CVS at the time of your scheduled appointment. In that circumstance we will either reschedule the appointment for 5-7 days time (when the placenta will be larger or the uterine position generally improved) or discuss the role of amniocentesis (15-16 weeks gestation).
Maternal contamination with decidual tissue is possible but rare with modern cytogenetic dissection techniques. For some molecular tests maternal and paternal blood will need to be taken for contamination studies: in such cases we recommend you attend Genetic Services of Western Australia for pretest counseling and ordering of these blood tests prior to your CVS appointment.
Although in general the placental chorionic villi have the same chromosomal appearance as the fetus, in 1-2% of cases there may be a discrepancy between the chromosomes in the chorionic villi and those of the fetus. This is called confined placental mosaicism, as the abnormality is typically present in the placenta and not the fetus. An additional amniocentesis at 15-16 weeks gestation may be required to clarify the situation in this uncommon circumstance.
