Reasons for testing

Understanding your testing options helps you prepare for the future.

Your reasons for testing may include:

the recommendation from The American College of Obstetricians and Gynecologists that all pregnant women be offered the option of diagnostic testing (as opposed to just screening) for Down syndrome, trisomy 18 and open neural tube defects
a risk for a known inherited disorder, birth defect or mental retardation
abnormal results from a maternal screening test, such as FirstScreen^®, SequentialScreen^SM, or ultrasound
a family history of chromosome abnormalities

If you are not sure whether you should have chorionic villi sampling (CVS), please talk with your doctor, a genetic counselor, or other specialist in genetics. See “Questions to Ask Your Doctor”.

Testing for...

Chorionic villi sampling (CVS) tests for:

Fetal chromosome abnormalities. The most common chromosome abnormality is Down syndrome, caused by an extra #21 chromosome. The next most common chromosome abnormalities are trisomy 18 and trisomy 13. Testing detects over 99% of all chromosome abnormalities.
Specific inherited diseases when parents have been shown to be carriers of the specific disease. Examples include Tay-Sachs disease, cystic fibrosis and sickle cell disease.

How the test works

Chorionic villi sampling (CVS) testing is a prenatal diagnostic procedure that is performed between the 10th and the 12th week of pregnancy. A small piece of the chorionic villi is removed from the placenta either through a needle inserted into the lower abdomen similar to amniocentesis or through a very thin tube inserted into the cervix. This sample is then sent to the laboratory for an analysis of certain birth defects and genetic disorders.

An ultrasound (sonogram) is used to guide the procedure. Most women report a slightly uncomfortable feeling, similar to a cramping or pressure sensation, rather than sharp pain, but every woman is different.

Before the procedure, the doctor performs an ultrasound to show a picture of the uterus, the placenta and the fetus on a monitor. Depending on the position of the placenta and fetus, the doctor uses one of two approaches to obtain the chorionic villi sample, either through the cervix (transcervical) or through the abdomen (transabdominal).

Transcervical – the vagina is held in an open position with a speculum. Then, a very thin tube is inserted through your cervix to reach the chorionic tissue.
Transabdominal – the doctor inserts a thin needle through your abdominal skin into your uterus.

A – Transcervical B – Transabdominal

Sometimes chorionic villi sampling (CVS) cannot be performed because of an infection, an unusually shaped uterus or an unusual fetal or placental position, for example. If CVS is not possible, amniocentesis can usually be performed at approximately 16 weeks.

What's not detected

Unlike amniocentesis, chorionic villi sampling (CVS) cannot be used to test for open neural tube defects, such as spina bifida and anencephaly.

If you are undergoing CVS, it is usually recommended that you also have the blood test called maternal serum alpha-fetoprotein (MSAFP) screening, at about 16-18 weeks of pregnancy. The MSAFP test identifies most (but not all) pregnancies at risk for open neural tube defects. An ultrasound at 16-18 weeks is also recommended to view the fetal anatomy.

Neither CVS nor amniocentesis is able to test for the kinds of birth defects that do not have a known cause. A few examples are autism, cleft lip, non-specific mental retardation and most heart defects. Every pregnancy has some risk, 3-5%, for these kinds of defects.

Understanding results

Since the chorionic tissue and the fetus originate from the same fertilized cell, they are usually genetically the same. However, in approximately 1 in 100 cases, the sample shows a mixture of normal and abnormal cells. In this case, a follow-up amniocentesis may be needed. There is also an estimated 1 in 1,000 chance that, even though the CVS results are normal, the fetus may have a chromosome abnormality. It is also possible that some maternal cells in the sample may interfere with the results. If your results are abnormal, you should talk with your doctor or with a genetics specialist about what your results mean.