Advances in medicine continue to improve our diagnostic capabilities. One of the new diagnostic tests, done on a simple blood sample drawn from the pregnant mother, has provided us with the ability to diagnose the presence of extra chromosomes in the developing baby with greater than 99% accuracy. This article will provide our patients with a clearer understanding of the specifics of the testing, the advantages of the test and who might want to consider the testing.
The test, at present, is mainly used to determine the presence of an extra chromosome at chromosomes 21, 13, or 18. Down Syndrome (DS) is the most common condition and is the result of an extra chromosome at number 21. The presence of this extra chromosome can cause varying degrees of mental retardation as well as various bodily changes such as facial and heart abnormalities. This condition is usually what patients request testing for.
Chains of DNA from the baby and placenta can be found in the mother’s blood in large enough quantities to properly count. Computerized counting of these DNA chains, (which are attached to each other in a manner that is specific to each chromosome), can demonstrate the presence of an excess number of specific chromosomes. The test will determine the presence of the extra chromosome 21 (Down Syndrome) with upwards of 99.5% accuracy. This makes the test an extremely reliable screening test for Down Syndrome which can be substituted for more invasive testing like amniocentesis and CVS which can cause miscarriage.
At present the main drawback to the test is the cost; thus the test is mainly reserved for women who are at higher risk of having a Down Syndrome infant. Our ACOG (American College of Ob-GYN,) recommends, and most insurance covers, testing for women who are 35 and older and women who have had a previous infant with chromosome abnormalities.
The patients who are at lower risk should recognize that there are other available screening tests which are covered by insurance, give a statistical risk of carrying a Down Syndrome baby, and can provide a great deal of reassurance. In addition, patients who would not terminate a pregnancy even if they were carrying a Down Syndrome baby might not even consider any screening.
Patients should also keep in mind that the farther along their pregnancy the higher the number of countable DNA chains. Although it is preferable to make a diagnosis as early as possible and most blood samples are adequate after 10 weeks of pregnancy, there are a small number of samples which are inadequate and the testing must be repeated a week or two later. Also women weighing more than 200 lbs are more difficult to retrieve adequate samples from.
This breakthrough in technology is a significant advancement and provides non invasive, extremely accurate testing on a simple blood sample from the mother. The retrieval of fetal DNA also opens many new opportunities to further evaluate the baby for other future life threatening diseases as well. It will be most interesting to see how this technique will impact future testing of the developing fetus.