There are two different kinds of birthmarks: vascular birthmarks and pigmented birthmarks. There are many kinds of vascular birthmarks and they are related to abnormal proliferations of blood vessels and vascular skin changes. Pigmented birthmarks are related to clusters of pigmented cells.
It is not exactly known why birthmarks occur but there are theories. One of these theories concludes that birth marks occur during fetal development because certain cells are placed in the wrong layer of skin.
Birthmarks are more common in females and they are considered to be non-hereditary.
There is no known reason why birthmarks are caused. They are not related in any way to what the mother ate or did during pregnancy. They are seldom inherited and are not related to any injury during childbirth.
There are mainly two types of birthmarks and they vary by the reason they occur. Vascular (blood vessel) birthmarks are a result of blood vessels not forming correctly, there are too many of them or they are wider than usual. Pigmented birthmarks are a result of an overgrowth of cells that create pigments in the skin.