There are three different versions of trisomy 13:

The most common version, full trisomy 13 is caused when there are three copies of chromosome 13 in each body cell.

The second version, mosaic trisomy 13, is caused when only a fraction of the individuals’ body cells contain three copies of chromosome 13. In mosaicism, the severity of the disease has a direct correlation to the number of affected cells. In other words, when more cells are affected, the disease is more severe.

The last variation of this disease is called partial trisomy 13. This is caused when part of chromosome 13 becomes translocated—or attached—to another chromosome. The resulting daughter cells will only have part of the extra chromosome.

Mosaic and partial trisomy 13 are much milder the latter, and usually have a better outcome on the individual’s life span. Unfortunately, however, these versions are also much less common.