Trisomy 16 is an unusual thing within a person where there are three copies of chromosome 16 rather than two. It often leads to women losing their baby in miscarriage. It is the second most common cause of miscarriage due to chromosomes (closely following X-chromosome monosomy). About 6% of miscarriages have trisomy 16. Those mostly occur in pregnancy from 8 to 15 weeks after the last menstrual period.
There are four types of Trisomy 16, "full trisomy 16", "translocation trisomy 16", "mosaic trisomy 16", and "partial trisomy 16":
- Full trisomy 16 or free trisomy 16 - all body cells have an extra copy of chromosome 16
- Translocation trisomy 16 - all body cells have an extra copy of the chromosome 16, but it moved close to another chromosome. In genetics, this movement is called translocation
- Mosaic trisomy 16 - Some body cells have three copies of chromosome 16, others have the usual two copies. In genetics, when there are different karyotypes, this is called a mosaic.
- Partial trisomy 16 - The body cells have two copies of chromosome 16, but part of the chromosome is copied. This makes chromosome 16 a little longer. For the part that is copied there are three copies of the information.
It is not possible for a child to be born alive with an extra copy of this chromosome present in all cells (full trisomy 16 or translocation trisomy 16). It is possible, however, for a child to be born alive with the mosaic form. Normally humans have two copies of chromosome 16, one inherited by each parent. This chromosome represents almost 3% of all DNA in cells.