Birthmarks occur due to localized overgrowths in the skin's structures present at birth or shortly after. They are caused by either an overgrowth of blood vessels (vascular birthmarks) or pigment cells (melanocytes, leading to pigmented birthmarks). The exact reason why these overgrowths happen is not fully understood, but they are generally not due to anything the mother did or did not do during pregnancy. Some birthmarks may have a genetic component, but most are not inherited. Types include red or purple marks caused by blood vessel clusters, and darker or brown marks caused by clusters of pigment cells. Most birthmarks are harmless, although a few can be associated with medical conditions or cosmetic concerns.
