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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. But they do sometimes rupture, twist, or bleed, which can cause severe pelvic pain.