A functional ovarian cyst is a sac that forms on the surface of a woman’s ovary during ovulation. It holds a maturing egg. Usually the sac goes away after the egg is released. If an egg is not released, or if the sac closes up following the release of the egg, the sac can swell up with fluid.
Functional ovarian cysts are different than ovarian growths caused by other problems, such as cancer. Most of these cysts are harmless. They do not cause symptoms, and they go away without treatment. But if a cyst becomes large, it can twist, rupture, or bleed and can be very painful.