A birthmark is a congenital, benign irregularity on the skin which is present at birth or appears shortly after birth. Usually in the first month. Birthmarks can occur anywhere on the body and take any shape. They are caused by overgrowth of blood vessels, melanocytes, smooth muscle, fat, fibroblasts, or keratinocytes.

They're often Located in places like the arm, legs, butt, chest, or head. But could show up anywhere on the body.

While they're often shapeless taking on a variety of looks, in fiction they could look like anything. A crown, heart, snake, spade, ring, whatever you can think of. They Can either be of a darker color of skin, or a different color entirely.

For more information on how birthmarks actually work please check Wikipedia.

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