An ovarian cyst is a fluid-filled sac that forms inside the ovary. The most common type is the functional cyst, which arises naturally during ovulation and usually disappears within 1 to 3 months. When a follicle fails to rupture just before ovulation, it becomes a follicular cyst; when the corpus luteum swells excessively after ovulation, it becomes a corpus luteum cyst. In contrast, there are non-functional cysts that do not resolve on their own, such as an endometrioma (chocolate cyst), in which endometrial tissue settles on the ovary and old blood accumulates; a dermoid cyst (teratoma), which contains fat and hair tissue; and serous or mucinous cysts. Depending on size, shape, and internal structure, evaluation may be needed to rule out malignancy.