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Weigh the Genetic Tests during Pregnancy

Without a doubt, the field of human genetics is the fastest growing field of medicine today. The Human Genome Project was sponsored by the U.S. government to sequence all 3 billion basic DNA segments (base pairs). It began around 1990 and it was anticipated that it would be done toward the end of the first decade of the new millennium. Scientists finished much earlier than they had anticipated — largely due to two reasons: First, the equipment to sequence the genome became better and more efficient over the years, so more DNA segments could be sequenced in a smaller amount of time. Second, scientists learned that the human genome contained far fewer genes than was originally thought.

Just as the knowledge about genes and disease has grown exponentially, so has the availability of testing for genetic disorders or traits. It used to be that deciding who should have an amniocentesis was as simple as knowing your patient’s age. If she was 35 or older, she was encouraged to have an amniocentesis to check the chromosomes of her baby. If she was under the age of 35, she wasn’t. With the development of maternal serum screening and nuchal translucency ultrasounds, providers were able to provide pregnancy-specific risk estimates of chromosomal abnormalities for all women, not just those women over 35. The patient could then use that risk to determine if she wanted to have an amniocentesis (or chorionic villus sampling) to find out definitively whether or not the chromosomes were normal.

Why doesn’t everyone have an amniocentesis or chorionic villus sampling, you might ask? Well, these procedures (sometimes called “invasive” because they “invade” the amniotic cavity) come with a risk. For some poorly understood reason, some women have a miscarriage after having these procedures — even if the procedure was done by an experienced person and went smoothly. The exact risk of this happening is difficult to assess, because it is impossible to separate the women who had a miscarriage after an amniocentesis or chorionic villus sampling because of the procedure from those who were predestined to have a miscarriage at that time in their pregnancy anyway (because women who don’t have an invasive procedure can have a miscarriage, too).

The best estimate, based on a perusal of all the contemporary medical literature, is that these procedures may actually cause a miscarriage about 1 in every 1,000 times they are performed. Don’t fall into the trap, however, of comparing your numerical risk from having a baby with a chromosomal abnormality to the estimated numerical risk of having a miscarriage after amniocentesis or chorionic villus sampling. The two numbers may be the same or close, but the meaning and implications of each of the possible outcomes (having a baby with Down syndrome or having a miscarriage as a result of having had an amniocentesis or chorionic villus sampling) vary widely from patient to patient and couple to couple. You should use these risk estimates as a starting point for your discussion with your significant other to find out which outcome would be the worst for your family.

If you’re up to your ears with numbers and risks but still can’t make a decision, see a genetic counselor. These professionals are specifically trained to help you through the decision process to find out what’s right for you and your family. They will take a careful family history to see if there are any disorders your children may be at risk for that you didn’t know about. They are also knowledgeable about all the different types of tests that are available and will help you choose from the menu of options.

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