Trisomy Awareness Month takes place in March.
What is trisomy?
Our genes, which are coded for in DNA, contain the information that makes us who we are.
In living cells, DNA is packaged into “chunks” called chromosomes. At the moment of conception, we inherit 23 chromosomes from our mother in the egg and 23 from our father in the sperm. That makes for a total of 46 chromosomes in every cell of our bodies. At least that is what happens most of the time.
However, sometimes, the egg or the sperm ends up with an extra chromosome. When this happens, the resulting embryo has an extra chromosome in every cell. This situation is called trisomy, and in principle, this can happen with any of the 23 chromosomes. Trisomy happens a lot more than anyone ever imagined. For most of the chromosomes, having an extra copy is incompatible with life. Most embryos with trisomy are lost (as miscarriages) during early pregnancy.
However, for certain chromosomes, having an extra copy can be compatible, not only with continued pregnancy, but also with long-term survival. The most common and best known example of is Down Syndrome (also called “Trisomy 21”), where there is an extra copy of chromosome number 21. It turns out that chromosome 21 is the smallest chromosome, representing only 2 percent of all of the DNA. This is probably why having an extra copy causes fewer developmental problems than occur when other chromosomes are present in an extra copy.
Although we know that Trisomy 21 results in the atypical development of a number of body structures, frequently including the heart and almost always including the brain, we still do not know why this atypical development occurs.
Why does trisomy happen?
The short answer is that we do not know. We do know that many types of trisomy, including Trisomy 21, increase in frequency with increasing maternal age. Like other cells, primary egg cells have 46 chromosomes, and there is something about being older that increases the chance that mistakes are made when the chromosomes are divided up among the final, mature eggs. After years of research, we still do not know why maternal age matters.
What about trisomy in pregnancy?
Although the chances of trisomy are highly dependent on maternal age, it can and does occur in young women, too. Because of this, we offer all expecting parents testing to see if their pregnancy may be affected. This testing is entirely voluntary and will never be performed without the mother’s consent.
If testing does show that the pregnancy is affected by trisomy, specialized counseling is available. For those parents who choose to continue a pregnancy affected by trisomy, every effort is made to help prepare them for the special needs that may arise in babies with Down Syndrome.
Stephen Brown, MD, specializes in gynecology, obstetrics, clinical genetics and clinical molecular genetics. His areas of expertise include reproductive genetics and prenatal diagnosis. Dr. Brown is an associate professor in the department of Obstetrics, Gynecology and Reproductive Sciences at the Larner College of Medicine at the University of Vermont.