Birth marks are blemishes on the skin that are formed while you are still in the womb. The exact cause is unknown, but we know they aren’t inherited, and that they are very common. I have one in my armpit!
They are a benign (that is, nothing to worry about) growth of blood vessels at the skin surface, and are thought to occur when the growth of those blood vessels goes wrong . However, they aren’t a sign of illness, and aren’t related to cancer.