Most ovarian cysts form as a normal part of your menstrual cycle, when a fluid-filled sac on the ovary doesn’t behave quite the way it should during ovulation. These are called functional cysts, and they’re so common that most people with ovaries will develop at least one during their reproductive years without ever knowing it. A smaller number of cysts develop from other causes, including conditions like endometriosis or unusual cell growth that has nothing to do with your monthly cycle.
Functional Cysts: The Most Common Type
Every month, your ovary grows a small fluid-filled sac called a follicle, which houses a maturing egg. When everything goes smoothly, that follicle ruptures, releases the egg, and is gradually reabsorbed by your body. A functional cyst forms when some part of this process stalls.
There are two ways this happens:
- Follicular cysts develop when the follicle never ruptures. Instead of breaking open to release the egg, it keeps accumulating fluid and grows into a cyst. Sometimes the egg is released through a different route, but the follicle itself stays intact and continues to expand.
- Corpus luteum cysts form after the egg has already been released. Normally, the empty follicle collapses into a small yellowish structure that produces hormones for a few weeks, then breaks down. But sometimes that structure reseals itself, fills with fluid, and grows into a cyst instead of shrinking away.
Both types are generally harmless. Most are smaller than 5 centimeters, cause no symptoms, and resolve on their own within one to three menstrual cycles. Simple cysts between 5 and 7 centimeters in premenopausal people are still usually physiological and tend to resolve within three months.
Cysts Caused by Endometriosis
Endometriomas, sometimes called chocolate cysts because of their dark brown contents, form when tissue similar to the uterine lining grows on or inside the ovary. The leading theory is that some menstrual tissue flows backward through the fallopian tubes during your period instead of leaving the body through the vagina. When that tissue lands on an ovary, it responds to your hormones just like the lining of your uterus would. It thickens, bleeds with each cycle, and has nowhere to drain. Over time, the trapped old blood and tissue form a cyst filled with thick, dark fluid.
These cysts don’t resolve on their own the way functional cysts do. They tend to grow slowly over many cycles and can cause significant pelvic pain, especially during periods or sex.
Dermoid Cysts: Present From Birth
Dermoid cysts are among the stranger things the body can produce. They form from germ cells, the same cells that eventually become eggs. During early development, germ cells contain three layers that are meant to grow into different types of tissue: one layer becomes skin, hair, and teeth; another becomes muscle and bone; and a third becomes organs like the thyroid. Sometimes these layers grow abnormally, with mature tissue bunching together into a cyst. The result is a growth that can contain hair, teeth, skin, and even fatty tissue.
Because they originate from cells present since fetal development, dermoid cysts can be carried for years or decades before being discovered, often incidentally during an imaging scan for something else. They grow slowly and are almost always benign.
Cystadenomas: Cysts From the Ovary’s Surface
Cystadenomas develop from cells on the outer surface of the ovary. Unlike functional cysts, which are related to the egg-release cycle, these are true growths. They’re benign and can be filled with either a thin, watery fluid (serous cystadenomas) or a thicker, mucus-like substance (mucinous cystadenomas). Mucinous types in particular can grow quite large. They don’t resolve on their own and typically need monitoring or removal depending on size.
How PCOS Leads to Multiple Cysts
Polycystic ovary syndrome creates cysts through a hormonal chain reaction. Higher-than-normal levels of androgens (hormones typically associated with male development) interfere with ovulation. Eggs begin to mature inside their follicles but never develop fully and are never released. The stalled follicles accumulate along the outer edge of the ovary, visible on ultrasound as a string of small cysts. Insulin resistance plays a role too, because elevated insulin can drive the body to produce even more androgens, making the cycle harder to break.
Despite the name, the “cysts” in PCOS are really just immature follicles that got stuck. They’re different from a single large functional cyst or a dermoid, and the condition is treated by addressing the underlying hormonal imbalance rather than the cysts themselves.
What Makes Some People More Likely to Get Cysts
Anything that affects ovulation can increase the chance of functional cysts forming. Hormonal fluctuations during puberty make girls older than 10 more susceptible, because surges in reproductive hormones drive follicle growth that doesn’t always complete normally. Fertility medications that stimulate the ovaries to produce multiple follicles at once can also raise the risk, since more follicles mean more opportunities for one to stall.
Endometriosis is the primary risk factor for chocolate cysts. A prior history of ovarian cysts makes recurrence more likely. And in early pregnancy, the corpus luteum sometimes persists longer than usual to support hormone production, which can allow it to fill with fluid and form a cyst.
When a Cyst Becomes a Problem
Most ovarian cysts cause no symptoms and disappear without treatment. But size matters. Cysts larger than 5 centimeters carry an increased risk of ovarian torsion, where the weight of the cyst causes the ovary to twist on its blood supply. Torsion is a gynecologic emergency that causes sudden, severe pelvic pain and typically requires surgery.
Cysts can also rupture, spilling their contents into the pelvic cavity. A ruptured functional cyst usually causes sharp, temporary pain and resolves on its own. A ruptured endometrioma can cause more intense inflammation because of its contents.
In premenopausal people, simple cysts under 7 centimeters are generally monitored rather than treated. Cysts larger than 7 centimeters are more likely to require surgical evaluation. For anyone who has gone through menopause, the threshold is lower: any cyst over 1 centimeter warrants further assessment, because functional cysts shouldn’t be forming once ovulation has stopped. Complex cysts, those with solid areas, internal walls, or irregular features on ultrasound, are investigated regardless of age or size.