Question

how are birth marks formed

Answer 1

There are two main types of birthmarks, which have different causes. Vascular (blood vessel) birthmarks happen when blood vessels don't form correctly — either there are too many of them or they're wider than usual. Pigmented birthmarks are caused by an overgrowth of the cells that create pigment (color) in skin.

Answer 2

Vascular birthmarks - a red, pink, or purple blemish - are caused by abnormal blood vessels under the skin. Some people are born with pigmented birthmarks; these are usually brown and are caused by the clustering of pigment cells