I am a massage therapist who's been practicing for 20 years, and I know that in these discussions, a lot has been said referring to this marking in Alzheimer's patients. Well, I'm not sure what the connection may be, but I've noticed it on half of my adult clients, young and old. I and my husband, brothers, etc also have it. I'm very interested to find out why it shows up there in countless people.
They're called birth marks because that's the latin translation for 'nevi' where the medical community doesn't say Foxglove, they say digitalis purpurea for the common type. And here digitalis the medicine is also latin which is likely to stick as a name.
Birthmarks are common and not marked at birth but already present before birth and therefore seen first after birth. There is no correlation between any of the marks present at birth and AD which is easy to understand in that the skin is actually an organ and AD does not migrate to organs or center in the brain related to any specific function.
Here is a non-message (no personal agenda) list of some types of birthmarks: