Ovarian cysts are fluid-filled sacs that form on the ovaries. Most ovarian cysts form as a result of the menstrual cycle and are called functional cysts. These cysts occur when the ovaries grow small cysts called follicles each month, which produce the hormones estrogen and progesterone and break open to release an egg when you ovulate. A monthly follicle that keeps growing is known as a functional cyst. Other types of cysts are much less common, such as dermoid cysts and cystadenomas, which can become large and move the ovary out of position. This increases the chance of painful twisting of the ovary, called ovarian torsion. Ovarian torsion may reduce or stop blood flow to the ovary.

The exact cause of most ovarian cysts is unknown. However, hormonal problems, endometriosis, and atypical cell reproduction can cause cysts like dermoids and cystadenomas to form. Polycystic ovary syndrome (PCOS), a condition that causes multiple cysts on the ovaries, pregnancy, and severe pelvic infection are also linked to ovarian cysts. Most ovarian cysts cause no symptoms and go away on their own. But a large ovarian cyst can cause pain, pressure, bloating, swelling, or changes in menstrual periods. Sometimes, ovarian cysts can become twisted or burst open, which can cause serious symptoms.