Each month during your menstrual cycle, a follicle grows on your ovary. The follicle is where an egg is developing. Most months, an egg is released from this follicle, called ovulation. If the follicle fails to break open and release an egg, the fluid stays in the follicle and forms a cyst. This is called a follicular cyst.
Another type of cyst occurs after an egg has been released from a follicle. This is called a corpus luteum cyst. This type of cyst often contains a small amount of blood.
Ovarian cysts are more common in the childbearing years between puberty and menopause. The condition is less common after menopause.
Taking fertility drugs can cause a condition in which multiple large cysts are formed on the ovaries. This is called ovarian hyperstimulation syndrome. The cysts most often go away after a woman's period, or after a pregnancy.
Functional ovarian cysts are not the same as ovarian tumors, or cysts due to hormone-related conditions such as polycystic ovary syndrome.