Vascular 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.

Where does your birthmark come from?

Causes of birthmarks

The occurrence of birthmarks may be inherited. Some marks may be similar to marks on other family members, but most are not. Red birthmarks are caused by an overgrowth of blood vessels. Blue or brown birthmarks are caused by pigment cells (melanocytes).

Why does a person have a birthmark?

Birthmarks generally result from an overgrowth of a structure that is normally present in the skin. For example, an overgrowth of blood vessels produces vascular birthmarks or haemangiomas; an overgrowth of pigment cells produces congenital naevi or moles.