A cyst is a fluid-filled sac. Ovarian cysts are caused by abnormal fluid production or cell growth within the ovary, which leads to swelling. Ovarian cysts are classified into two groups: those that go away on their own, and those that need treatment. Fortunately, most ovarian cysts will subside by themselves. Even within the group that needs treatment, these cysts are usually benign. They're often detected during a pelvic exam, as a soft, movable lump. Large cysts may cause abdominal pain or pressure or irregular bleeding, and throw off hormonal balance. Other symptoms include painful intercourse or a swollen abdomen. However, cysts may produce no symptoms at all. If severe pain results, see a doctor immediately. While rare, this could signal a twisted ovarian cyst, which can damage the ovary if not treated promptly. When a cyst shows signs of cancer, is very painful, or doesn't go away in a couple of months, surgery is generally required. When small, the cyst alone can often be removed. Even with some large cysts, a portion of ovarian tissue can usually be saved. Cysts bigger than five inches in diameter may destroy the ovary, requiring its removal. Women who are prone to ovarian cysts may reduce the risk of recurrence by taking birth control pills. Ovarian cysts can be a complex health issue, so for complete information, be sure to consult a health care provider.