Functional ovarian cyst

An ovarian cyst is a fluid-filled sac (follicle) that contains a maturing egg. It forms on the ovary's surface, then disappears after it releases the egg. If a mature egg is not released or if the sac seals shut after releasing the egg, the sac can swell up with fluid. This forms a functional ovarian cyst.

Most functional ovarian cysts do not cause symptoms. And most go away by themselves. But they do sometimes break open (rupture), twist, or bleed, which can cause severe pelvic pain.