IPA: /ˈbɜːrθmɑːrk/
KK: /ˈbɜrθˌmɑrk/
A mark or spot on the skin that a person is born with, which can be a different color or texture than the surrounding skin.
She has a small birthmark on her arm that looks like a star.
Birthmark is formed from "birth" (from Old English "berthan", meaning to bring forth or give life) and "mark" (from Old English "mearc", meaning a boundary or sign). The word "birthmark" refers to a mark or sign that is present at the time of birth.
Think of 'bringing forth' ('birth') and a 'sign' or 'mark' that indicates something special about a person — that's why a birthmark is a mark present at birth.
No commonly confused words.