There are three types of trisomy 18:
Full trisomy 18. The extra chromosome is in every cell in the baby's body. This is by far the most common type of trisomy 18.
Partial trisomy 18.The child has only part of an extra chromosome 18. That extra part may be attached to another chromosome in the egg or sperm (called a translocation). This type of trisomy 18 is very rare.
Mosaic trisomy 18. The extra chromosome 18 is only in some of the baby's cells. This form of trisomy 18 is also rare.