Mosaic Trisomy 18 is different from full Trisomy 18 in that there are two different cell lines. One cell line has the normal 2 chromosomes and the other has the additional chromosome. How this happens is that in the beginning everything was fine, but somewhere along the line there was a copying mistake so that there are some normal cells and some with the extra chromosome. In my case, 70% of my cells have an extra chromosome.
Mosaic individuals tend to have a larger range of abilities. Some children are severely affected. Other individuals didn't even realize they had it until they were tested as adults, and are living very productive lives. It depends on which organs are affected. It is very hard to determine an accurate percentage because different parts of your body could carry a different percentage. How I describe it is that if you put your hand in a bag of M&M's, you could get one combination, but if you put your hand in a different area, you could get a different combination. To be accurate, you would have to test every area of your body. There are a number of mosaic children who had been tested as full Trisomy 18 when they were born, but later were diagnosed with mosaicism. Unfortunately, many parents who have a child with Mosaic Trisomy 18 are told that their child is "Incompatible with Life". This is absolutely false and this narrative needs to be changed. If you are a parent whose child has been recently diagnosed, please make sure that your medical team is willing to treat your child for their symptoms and not their label.