Birthmarks are simply clumps of blood vessels close together.
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The term "birthmark" can refer to all sorts of spots. One common type is the "hemangioma," which results from the clumping of several blood vessels in one spot. "Port-wine stains" are one instance of these harmless blotches. Others are bright red bumps (the "strawberry hemangioma"), and most disappear before adulthood.
Another common mark is the mole, which tends to appear in childhood (long after "birth"). A mole is a skin lesion containing high concentrations of the cells that produce melanin, a dark pigment that helps guard against damaging ultraviolet rays.
Depending on the amount and distribution of melanin, these can come in many different shapes and colors, so don't worry if you have a few different looking moles. But if you have an abundance of beauty marks, or notice any changes in mole size or color after puberty, get checked by a dermatologist. 600,000 people in the United States alone are diagnosed each year with malignant neoplasms. Whatever you call it, "birthmark", "hemangioma" and "mole" all sound better than "cancer."