Ovarian cysts are fluid-filled sacs in or on an ovary.
Functional cysts are usually smaller than abnormal cysts.
The most common functional cyst is a follicular cyst. This type of cyst forms when the egg-producing follicle of the ovary enlarges and fills with fluid. The other functional cyst is the corpus luteum cyst, which occurs after the egg has been released from the follicle. When pregnancy does not occur, the corpus luteum usually disintegrates. However, occasionally it swells with fluid or blood and remains on the surface of the ovary as a cyst.