Chorionic Villus Sampling (CVS)
Chorionic villus sampling (CVS) is used to help diagnose chromosome abnormalities such as Down syndrome. It is offered when a pregnancy is found to have an increased chance of a chromosome abnormality, such as through a positive result in a first trimester screen for Down syndrome, or when a woman is 35 years or older at delivery.
CVS can also be used to test for other genetic conditions. This is offered only when a genetic counsellor and geneticist identify a risk for a specific genetic condition.
CVS is done twice a week at Mount Sinai on an out-patient basis.
A few cells from the developing placenta (the chorionic villi) are extracted instead of amniotic fluid cells. The sample is obtained either by passing a small catheter through the vagina and cervix into the uterus, or by inserting a fine needle through the abdominal wall.
The procedure takes approximately 20 minutes with a resting period of 20 minutes after.
Risks / Limitations of CVS
The natural pregnancy loss after about 9 to 11 weeks is about 4%. The additional loss due to CVS is about 1 per cent, bringing the total loss rate risk to about 5%. At the time of the clinic visit an ultrasound examination will be performed to determine the size of the fetus. In about 10% of women, examination prior to CVS will show the fetus is no longer alive.
Injury to baby
There have been a few reports in the literature suggesting an association between CVS and certain types of limb abnormalities in the fetus. In 1992 the WHO (World Health Organization) reported no significance in the incidence of limb abnormalities in women having CVS compared with the incidence in the general population when the CVS is performed by experienced physicians at 10 weeks of pregnancy or later.
Spotting or cramping
Some patients having CVS may have some spotting or bleeding following the procedure. This usually does not last more than a couple of days. However, if bleeding is heavy or persistent call your physician or go to the emergency room of the hospital close to you.
Infection following the procedure is extremely rare. However if you develop fever, have chills, cramps or bleeding call your doctor immediately.
In our clinic about 6% of patients have an amniocentesis following CVS. This may be because it was not possible to obtain an adequate CVS sample or because of difficulty interpreting the CVS results.
What isn't detected by CVS?
Unlike amniocentesis, CVS cannot detect spina bifida. However, a blood test called Maternal Serum Screening (MSS) should be done at about 16 weeks of pregnancy and can be arranged by the woman's obstetrician or family physician or by our program.
Accuracy of CVS results
The cells taken at CVS come from the developing placenta and originate from the same fertilized egg as the fetus. The chromosome findings in the chorionic villi are usually the same as in the fetus. However, abnormal cells may be found in the placenta which are not found in the fetus. This is called "mosaicism". Hence accuracy of CVS is 98% which is less than that of amniocentesis.
The major advantage of CVS is timing. The test can be done as early as the 10th week of pregnancy, and results can be obtained by the 13-14th week of pregnancy.
Chromosome problems such as Down Syndrome can be detected by CVS. In addition, obvious chromosome rearrangements can be detected by CVS. In cases where there is a family history of a specific genetic condition, special genetic testing can also be ordered on the chorionic villus sample in addition to routine chromosome testing.
Does CVS guarantee a normal baby?
CVS cannot provide information about all aspects of a baby's development, for example mental retardation. For every pregnancy, regardless of the mother's age, there is a chance that a baby will have a birth defect that cannot be detected before birth. No test exists which will guarantee a normal baby.
What if I am Rh negative?
Rhogam immunoglobulin injections are given at the time of the procedure to mothers with Rh negative blood type to prevent development of antibodies which could harm the baby.