Chorionic villus sampling (CVS) is used to 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 screen positive result in first trimester screening, 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.

A sample from the developing placenta (the chorionic villi) is obtained. 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 30 minutes after.

The sample obtained is from the developing placenta and originates from the same fertilized egg that formed the fetus. The chromosome findings in the chorionic villi are usually the same as in the fetus. However, in about 2% of cases, abnormal cells may be found in the placenta that are usually not in the fetus. This is called “mosaicism”. Hence the accuracy of CVS is 98% which is less than that of amniocentesis.

This test is usually done at Mount Sinai Hospital between 11-14 weeks gestation. However, it can also be done later using a transabdominal approach.

Results can be obtained by the 13-14th week of pregnancy. However, at Mount Sinai Hospital, using a technique called QF-PCR, we can provide information regarding chromosomes 13, 18, 21, X & Y and the gender of the baby within 24-48 hours.

In addition to detecting Down Syndrome, CVS can detect other obvious chromosome abnormalities. In cases where there is a family history of a specific genetic condition, special genetic testing can also be ordered in addition to routine chromosome testing.

The accuracy of CVS is 98 percent (98%), which is less than that of amniocentesis.

The major advantage of CVS is timing. The test can be done as early as the 11th week of pregnancy, and results can be obtained by the 13-14th week of pregnancy.

Miscarriage: The natural pregnancy loss after about 9 to 11 weeks is about 4 per cent. The additional loss due to CVS is about 1 per cent. At the time of the clinic visit an ultrasound examination will be performed to determine viability and the size of the fetus as well as the location of the placenta. In about 10 per cent 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 World Health Organization reported that the incidence of limb abnormalities in women having CVS is not different than the general population when the CVS is performed by experienced physicians after 10 weeks of pregnancy.

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 nearest emergency room.

Infection: Infection following the procedure is extremely rare. However if you develop fever, chills, cramps or bleeding call your doctor immediately or go to the nearest emergency room.

Unlike amniocentesis, CVS cannot detect open spina bifida. However, a blood test called Maternal Serum Screening (MSS) can be done at about 16 weeks of pregnancy and can be arranged by your obstetrician, family physician or by our program.

CVS, as well as amniocentesis, 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.

In some cases repeat testing may be necessary because it was not possible to obtain an adequate CVS sample or because of difficulty interpreting the CVS results.

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.