Ovarian cysts are quite common in women during their childbearing years. An ovarian cyst is a fluid-filled sac which develops on the ovaries. 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. A woman can develop one cyst or many cysts. Ovarian cysts can vary in size – from as small as a pea to as big as a water melon.