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An ovarian cyst is a fluid-filled sac (follicle) that contains a maturing egg; it forms on the ovary's surface, then disappears not long after it releases the egg. If a mature egg is not released or if the sac reseals after releasing the egg, the sac can swell up with fluid, forming a functional ovarian cyst.
Most functional ovarian cysts do not cause symptoms and go away by themselves. However, they do sometimes rupture, twist, or bleed, which can cause severe pelvic pain.
WebMD Medical Reference from Healthwise