They are not contagious and almost never hereditary. More girls than boys appear to be affected. Several instances of identical twins, one with a large mole and the other without, are known. Having one child with a large nevus slightly increases the risk of having further children with the condition. A person with a large nevus has a slightly increased risk of having children with a nevus.
Exact risks are unknown but very low.
In our support group to date, only one family has 2 members with a large nevus and no one with a large nevus has had a child with one. Giant congenital nevi involving much of the body surface are less common, possibly around one in every 250,000 to 500,000 births. There has been no report to date of a melanoma occurring in a satellite nevus of a person with a large nevus. Melanocytes are special cells that give human skin its color. They are thought to come from an area near the brain and spinal cord in the developing baby and to migrate to the skin before birth.