March is trisomy awareness month. Trisomy is a genetic abnormality that results from improper distribution of chromosomes during cell division. This improper division is called non-disjunction. Trisomy is when a cell has an extra chromosome. Complete trisomy of chromosome 8 is fatal, and usually results in a miscarriage during the first trimester of pregnancy, although this is not the case for other trisomies. It is important to note that most trisomy occurrences are not a result of inheritance, but are rather a genetic accident.
Which is the most common?
Trisomy 21 is the most common, also known as Down Syndrome. Other less common trisomies can occur on chromosome 8, 13 (Patau syndrome), and 18 (Edwards syndrome), but there are others.
What is the difference between partial, mosaic, and complete trisomy?
Partial trisomy is when only a piece of the extra chromosome is in the cells. Mosaic trisomy occurs when only some of the cells have the mutation of an extra chromosome. Complete trisomy is when all the cells in the body have an extra chromosome.
What is the prognosis for someone with trisomy?
Symptoms range from mild to severe, and lifespan depends heavily on what chromosome is affected, and what symptoms occur. Just to show how vast the differences are between the syndromes, let’s look at the average lifespan between a few of them.
Down Syndrome: 60 years
Edwards’ syndrome: around a year if mosaic trisomy, fatal if complete trisomy.
Patau syndrome: 7-10 days
Trisomy 8: can live average lifespan if mosaic, fatal if complete
How to test for trisomy
Screening for trisomy can begin as soon as 10 weeks. The test given screens for Downs syndrome, Edwards syndrome, and Patau syndrome. The test is performed by a blood test as well as a scan of nuchal translucency (fluid behind the fetus’ neck). This test assesses the risk of developing a trisomy.
Determining if the fetus has a trisomy involves invasive procedures that run the risk of miscarriage. These tests either sample cells from the placenta (chorionic sampling) or from the amniotic fluid (amniocentesis). Other diagnostic tools are scans done later in pregnancy which can reveal physical abnormalities.