Scientists have been studying trisomies since chromosomes were first seen under a microscope. While trisomies have been studied and described for decades, there is still a lot that we don’t understand about the cause of trisomies. This article is going to cover some of the facts about what we do know, and provide some information about other common human trisomies.
Down syndrome is the most common liveborn chromosome abnormality in humans, but did you know that almost any chromosome can be seen in trisomic form? While in theory all chromosomes can be seen in trisomic form, in reality, very few trisomies are compatible with life. Some chromosomes - 13, 18, X and Y- are seen in liveborn children. While trisomies of chromosomes 15, 16, and 22 are often seen in miscarriages. The other chromosomes - 1-12,14,17,19 and 20 are almost never seen as full trisomies. It is thought that having these chromosomes in trisomic form, causes a very early miscarriage, before it can even be evaluated or treated.
Chromosomes and Miscarriage
Chromosomal anomalies are known to be the single most common cause of miscarriages. Historically, about 50% of miscarriages were thought to be due to abnormal chromosomes. However, now that pregnancy is detected earlier and we know that miscarriage is more common than previously thought, it is now suspected that the percent of miscarriages due to chromosome abnormalities is actually higher than 50%.
In miscarriages, the majority of chromosomal anomalies (95%) are due to an abnormal number of chromosomes. About 60% are due trisomies and a further 20% are found to be missing a sex chromosome - a condition called Turner syndrome or 45,X. Another common chromosome problem seen in miscarriages is a condition called triploidy. In triploidy, the fetus receives a full extra set of chromosomes for a total of 69 chromosomes. Most of the other human trisomies are so severe that they cause an early miscarriage or the baby has such severe problems, they die in infancy.
Trisomy 21 and Miscarriage
While Down syndrome is the most common trisomy seen in liveborns, it too carries a significant risk of miscarriage. Approximately 25% of fetuses with trisomy 21 will miscarry or be stillborn. Other trisomy seen in liveborn infants are include trisomy 13 and trisomy 18 which are much rarer than Down syndrome, but are much more severe.