What causes an ovarian cyst?

There are many different types of ovarian cysts.

Functional cysts:
A largest proportion of cysts in premenopausal women are “functional”, meaning that they are derived from the follicles that normally make up the ovary. Each follicle is made up of an egg surrounded by layers of cells. During a normal menstrual cycle, one follicle develops a collection of fluid thereby forming a “cyst” that grows to a size of around 2 cm. These “cysts” that form during every menstrual cycle are actually normal pre-ovulatory follicles that rupture to release eggs (ovulation). For more information on ovarian follicles and follicle development during the menstrual cycle, see my section on The Menstrual Cycle and Ovulation Cycle Tracking.

Sometimes these large follicles that are part of the normal menstrual cycle don’t disappear as they should. Instead, they persist and can become larger than 2-3 cm. These persistent and enlarged follicular cysts are called functional cysts. Functional cysts are common in women of reproductive age and 70-80% will resolve without any intervention.

Endometriomas or “Chocolate cysts”:
Endometriotic deposits on the ovary sometimes lead to the ovary becoming stuck to the pelvic side-wall through scarring. The deposit then becomes sandwiched between the ovary and pelvic wall and if it undergoes repeated episodes of bleeding, blood can accumulate in this space to form a cyst. After time, the altered blood takes on a chocolate-like appearance hence the name “chocolate cyst”. For more information, see my section on Endometriosis.

Neoplastic cysts:
Neoplastic cysts are tumours derived from overgrowth of particular populations of cells in the ovary. They can be benign or cancerous (malignant). The commonest benign neoplastic cysts are the serous and mucinous cystadenomas. Cancerous ovarian cysts, or ovarian carcinomas, are relatively rare and occur at the rate of around 15 cases per 100,000 women per year.

Dermoid cysts (Teratomas):
Dermoid cysts are benign and account for more than 10% of non-cancerous ovarian tumours. These are very unusual cysts that often contain different types of tissue such as hair and teeth.