Definition of Ovarian cysts:

An ovarian cyst is a sac filled with fluid that forms on or inside of an ovary.

Causes, incidence, and risk factors:

Ovarian cysts may occur during the process in which an egg is released from the ovary (ovulation). During the days before ovulation, a follicle grows. But when ovulation is supposed to occur, the follicle fails to break open and release an egg, as it is supposed to do. Instead, the fluid stays in the follicle and forms a cyst.

Ovarian cysts are somewhat common, and are more common during a woman's childbearing years (from puberty to menopause). Ovarian cysts are rare after menopause.

No known risk factors have been found.

Functional ovarian cysts are not the same as ovarian tumors (including ovarian cancer) or cysts due to hormone-related conditions such as polycystic ovary disease.