An ovarian cyst is any collection of fluid, surrounded by a very thin wall, within an ovary. Any ovarian follicle that is larger than about two centimeters is termed an ovarian cyst. An ovarian cyst can be as small as a pea, or larger than a cantaloupe.

Ovarian cysts are very common. Each primordial follicle or egg cell is a potential cyst. Quite a few things can happen to cause a primordial follicle to develop into a cyst:

1. The follicle does not rupture and the egg cell is not released. In this case the follicle grows bigger and bigger and become a cyst. This kind of follicle can become very large.
2. The corpus luteum can grow abnormally large and bleeding can occur in the cavity inside corpus luteum and cause it to become a large cyst.
3. The cells surrounding the egg cell might began developing spontaneously and cause abnormal cyst formation. Some of these cyst can become cancer.
4. The egg cell may start to develop on its own and because it contains only half ( 23 instead of 46) of the human chromosomes abnormal growths and cyst formation can occur. (also known as a cystic teratoma or dermoid cyst)
5. Endometriosis can occur on the ovaries. It can infiltrate the ovarian tissue and can cause cysts.