Amniocentesis (amnio) is a test that identifies the presence of a chromosomal or genetic defect in the growing fetus, such as Down Syndrome or Tay-Sachs disease.

But amniocentesis can’t be done until a woman is between 16 and 20 weeks pregnant. If a serious problem is found, and the patient chooses not to continue with the pregnancy, then it is a more involved procedure than during the first trimester. And it can be more heart wrenching to have to make such a decision after investing so much hope in a pregnancy that has progressed to 18 or 20 weeks.

What many prospective parents don’t realize is that there is an alternative to amniocentesis that can be done during the first trimester, as early as 10 weeks. It’s called chorionic villus sampling (CVS). It’s about as safe as amniocentesis. Either procedure (CVS or amnio) has a fetal loss rate (miscarriage) of less than 1%. And, in most cases, CVS can be as good a choice or better.

As one of the investigators for FDA trials of the procedure in the mid-1980s, I was among the first physicians in the U.S. to gain significant experience in chorionic villus sampling.

If you are older than 35 and your ob-gyn has recommended prenatal genetic testing but hasn’t offered this alternative to you, it may be because fewer physicians are trained in CVS than in amnio.

But you and your pregnancy should not be penalized just because your physician does not perform CVS.

If you’d prefer to have chorionic villus sampling, check with one of the major academic medical centers in your area where CVS is more likely to be performed regularly. And be sure to have it done by a physician who has over 100 procedures under his or her belt, because the procedure has a steep learning curve. The miscarriage rate is directly related to the experience of the person performing CVS.

The advantage with CVS is that you will know the results of the test within a week, while you are still in your first trimester.

- Yvonne Thornton, MD, MPH