It’s common to suddenly notice a bruise without remembering how it happened. Bruises form when “the blood vessels under your skin break and start to leak.” Most are harmless, but frequent or slow-healing bruises may need medical attention.

With age and sun exposure, skin thins and capillaries become fragile, leading to “unsightly purple patches, especially on the forearms and hands.” Dermatologists call this actinic (solar) purpura. Women, who “typically bruise more easily than men,” often notice it sooner. Sunscreen, long sleeves, and moisturizers can help.

Iron deficiency is another cause, especially in women with heavy periods. If tests confirm low iron, supplements and diet changes may help, but “sudden changes in periods are also cause for concern.”

Sometimes bruises come from unnoticed knocks—“thighs and shins” often hit furniture. For some, even small bumps leave big marks.

Genetics, fair skin, alcohol, and aging all increase bruising. Keeping a “photo diary” of bruises can help doctors spot patterns that need testing.