Functional ovarian cysts are common and considered a normal part of the menstrual cycle, and because of that, "approximately 1 in every 10 women will have an ovarian cyst at some point in their life," Taylor said. "Having had one sees you more likely to have another." Functional cysts can form from either follicles or the corpus luteum . To understand how and when these cysts form, let's first review the phases of the menstrual cycle: Each ovary contains egg follicles — tiny sacs that contain immature eggs (oocytes). At the beginning of each menstrual cycle, several of these follicles start to mature and grow. One of the mature follicles eventually bursts open and releases an egg, during ovulation. After releasing the egg, the empty follicle becomes a temporary, hormone-secreting structure called the corpus luteum. If the egg isn't fertilized, menstruation begins. The corpus luteum then breaks down and stops producing hormones, and estrogen and progesterone levels fall, causing the lining of the uterus to shed. Follicular cysts occur when a mature follicle doesn't burst to release an egg and instead grows into a fluid-filled cyst. Meanwhile, corpus luteum cysts form when the corpus luteum doesn't shrink as it should at the end of the menstrual cycle but instead reseals and builds up with fluid. These functional ovarian cysts tend to go away in a few weeks and usually cause no symptoms. These typically grow to around 1.2 inches (3 cm) , although corpus luteum cysts may, on occasion, grow up to 4 inches (10.2 cm) wide. "As these cysts form as part of the menstrual cycle they are most commonly seen in women who are between puberty and the menopause," Taylor said. "These functional cysts are typically benign and will resolve on their own." Pathological cysts tend to be larger than functional cysts. They include ovarian dermoid cysts , also known as teratomas, which are benign tumors that often form before birth and can contain tissue, such as hair, skin, teeth or fat. Dermoid cysts form when germ cells — stem cells which are supposed to become eggs — go rogue and start developing into other tissue types instead. They also include cystadenomas, which are cysts that develop from ovarian tissue and fill with a watery or mucous fluid. Cystadenomas can sometimes grow to about 3.9 inches (10 cm) across , on average, but some have been found to be as large as 11.8 inches (30 cm) in diameter. Conditions like endometriosis, which occurs when tissue similar to the uterine lining grows outside the uterus, can result in the formation of cysts named endometriomas. These form when uterine-lining-like tissue grows on the ovaries. These are often named "chocolate cysts," due to the fact that they contain thick, old blood, and they can cause painful periods, pelvic pain, pain during sex, and fertility issues. Around 10% of people who menstruate have endometriosis, and an estimated 17% to 44% of people with endometriosis may develop an endometrioma. Another condition, named polycystic ovary syndrome (PCOS), is caused by high levels of androgens (male sex hormones) and can result in the formation of several tiny cysts that measure only a few millimeters across. These form around the edge of the ovary as a result of underdeveloped follicles not maturing enough to release an egg. About 6% to 13% of reproductive-age women are estimated to have PCOS.