Q. When I was 10 weeks pregnant recently, I underwent prenatal testing by chorionic villus sampling -- CVS. My insurance company rejected payment for this procedure, saying it was considered experimental.

I knew the procedure was relatively new, but experimental?

My other option was amniocentesis, which is done at 15 to 17 weeks of pregnancy. This procedure would have been covered by insurance.

Because I wanted to know the results of the procedure earlier in pregnancy, I decided to have the CVS test.

Is CVS testing really experimental? How does it compare to amniocentesis?

A. CVS testing has come a long way in becoming a safe and effective procedure. In 1987, a panel of 31 experts was asked about the safety and effectiveness of this technique. They were evenly split over whether this was a proven or experimental procedure. And only about one in five thought its safety was proven, while around 70 percent thought further study was needed.

With more physicians gaining more experience with CVS testing, the picture has changed. A second panel of 42 experts was asked the same two questions 2 1/2 years later. Nearly two out of three thought that the safety of CVS testing was proven, and another third thought it was "promising." Only three physicians thought its safety was still unproven. Four out of five physicians thought CVS had been proven effective.

Chorionic villus sampling differs from amniocentesis in several ways. With amniocentesis, a doctor inserts a needle through the abdomen and into the fluid surrounding the fetus in the womb. Some of this fluid is drawn out, and the cells contained in it are grown and tested for genetic defects.

With CVS testing, a shielded needle is inserted through the opening to the womb in the cervix. A tiny piece of the placenta is taken for testing. Its cells contain the same genetic information as the fetus's.

The CVS procedure causes little pain and takes 10 to 15 minutes to perform. Like amniocentesis, it's done as an outpatient, so you can go home shortly after having it.

You're right that amniocentesis is done around 15 to 17 weeks of pregnancy. CVS testing is done between nine and 12 weeks. The risk of a miscarriage from either of these procedures is small, less than one in 100. Overall, amniocentesis has a slight edge on safety, by a fraction of a percent.

Both procedures are highly successful. Amniocentesis gives accurate information about 99 percent of the time and CVS testing about 98 percent.

These tests are very reliable in detecting genetic defects such as Down syndrome, hemophilia or sickle cell anemia or metabolic conditions such as Tay-Sachs disease.

CVS testing isn't right for all women, however. Several problems can make the test difficult or risky to perform, including infection of the cervix (by herpes or gonorrhea, for example), vaginal bleeding, fibroid tumors of the uterus, or a cervix that's bent, making it difficult to pass the sheathed needle.

For these women, trans-abdominal CVS testing is an option. In this procedure, a small piece of the placenta is taken through a needle inserted in the lower abdomen. This test, which can also be done anytime between nine and 12 weeks, is an alternative to the vaginal method. About half of CVS procedures currently done use this approach.

CVS testing is done at several medical centers in the Washington area. The Genetics & IVF Institute is one of the centers with considerable experience doing this procedure. It is located at 3020 Javier Rd., Fairfax, Va. 22031 (703-698-7355).

Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington.

Consultation is a health education column and is not a substitute for medical advice from your physician.

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