ovarian cysts: Ovarian cysts are fluid filled sacs on the surface of the ovary that are quite common in women during their childbearing years. Most cysts result from the changes in hormone levels that occur during the menstrual cycle and the production and release of eggs from the ovaries. Most are harmless and go away on their own. Your doctor will want to keep track of any cyst to be sure that it does not grow and become cancerous.