Most ovarian cysts form because of your normal monthly cycle. Each month, your ovaries grow a small fluid-filled sac (a follicle) to release an egg, and sometimes that process doesn’t go as planned. The follicle keeps growing, fills with fluid, and becomes a cyst. These “functional” cysts are extremely common and usually harmless. Even among women over 55, about 14% have a simple ovarian cyst at any given time, with roughly 8% developing a new one each year.
Less commonly, cysts form from conditions unrelated to ovulation, including endometriosis, abnormal cell growth, or infection. Understanding the different causes helps clarify why some cysts disappear on their own while others need monitoring or treatment.
Functional Cysts: The Most Common Type
Functional cysts come in two varieties, and both trace back to the same monthly process. During a normal cycle, a follicle in the ovary grows, matures an egg, and then ruptures to release it. If any step in that sequence stalls, a cyst can form.
Follicular Cysts
A follicular cyst develops when a follicle fails to rupture and release its egg. Instead of breaking open at ovulation, the follicle stays intact and continues filling with fluid. This is the single most common type of ovarian cyst.
The usual reason for the failed rupture is a hormonal timing problem. Ovulation requires a surge of luteinizing hormone (LH) from the pituitary gland. If that surge is too weak or mistimed, the follicle never gets the signal to pop open. Several things can disrupt that hormonal signal: chronic stress raises cortisol, which interferes with the feedback loop between your ovaries and brain, suppressing the LH surge. Irregular signaling from the hypothalamus (the brain’s hormonal control center) can also blunt LH release. Even exposure to extra progesterone during the wrong phase of the cycle can block the pituitary from releasing enough LH to trigger ovulation.
Follicular cysts typically resolve on their own within one to three menstrual cycles as hormone levels normalize.
Corpus Luteum Cysts
After a follicle successfully releases an egg, the empty sac is supposed to shrink and produce hormones to support a potential pregnancy. This leftover structure is called the corpus luteum. Sometimes, though, the opening where the egg exited seals shut. Fluid accumulates inside, and the corpus luteum swells into a cyst. Corpus luteum cysts can grow several centimeters and occasionally cause sharp pain, especially if they bleed internally or twist. Like follicular cysts, they usually disappear on their own.
How PCOS Causes Multiple Cysts
Polycystic ovary syndrome (PCOS) creates a different pattern. Instead of one rogue follicle, the ovaries contain many small follicles that start maturing but never finish. The result is a ring of tiny cysts visible on ultrasound, sometimes described as a “string of pearls.”
The core problem in PCOS is elevated androgens (hormones like testosterone that are typically present in smaller amounts in women). High androgen levels interfere with the normal follicle maturation process. Follicles begin to develop each cycle but stall before they’re large enough to ovulate. They accumulate over time, and without regular ovulation, the hormonal imbalance reinforces itself. PCOS affects roughly 1 in 10 women of reproductive age and is the leading cause of irregular periods and ovulation-related infertility.
Endometriomas: Cysts From Endometriosis
Endometriomas, sometimes called “chocolate cysts” because of the dark, old blood they contain, form when tissue similar to the uterine lining grows on or within the ovary. There are two main ways this happens.
In the first, a normal functional cyst ruptures or develops a small perforation. Endometrial tissue invades through that opening and colonizes the cyst lining. As the invading tissue advances, the original cyst lining regresses and is replaced. Each menstrual cycle, this tissue bleeds internally, and the trapped blood accumulates inside the cyst.
In the second pathway, endometrial tissue on the ovary’s surface bleeds directly into the ovarian tissue or folds inward, creating a small, densely scarred cyst from scratch. These tend to be smaller and more fibrotic than the invasion type. Either way, endometriomas are a hallmark of ovarian endometriosis and rarely resolve without treatment.
Dermoid Cysts and Other Growths
Dermoid cysts (technically called mature cystic teratomas) are among the stranger things the body can produce. They form from germ cells, the same cells that would normally become eggs. Germ cells carry the blueprint for all three layers of human tissue, which means dermoid cysts can contain skin, hair, sweat glands, and even teeth. They grow slowly, are present from birth, and are almost always benign. Many people don’t know they have one until it’s discovered incidentally on an imaging scan.
Cystadenomas are another non-functional cyst type. These develop from cells on the outer surface of the ovary and can fill with watery or mucous-like fluid. They tend to grow larger than functional cysts and sometimes need removal to prevent complications.
Fertility Medications and Cyst Risk
Fertility drugs that stimulate ovulation can significantly increase the chance of developing ovarian cysts. These medications work by amplifying hormonal signals to the ovary, encouraging multiple follicles to mature at once. That overstimulation sometimes causes follicles to enlarge without rupturing, or leads to overall ovarian swelling.
In clinical trials involving over 8,000 women taking clomiphene (a widely used ovulation-stimulating drug), 13.6% experienced ovarian enlargement. Cysts that form during fertility treatment typically shrink on their own after the medication is stopped, but treatment is usually paused until the ovaries return to normal size before another cycle begins.
Infection as a Cause
Severe pelvic infections can spread to the ovaries and trigger cyst formation. Pelvic inflammatory disease (PID), usually caused by sexually transmitted bacteria, is the most common pathway. When infection reaches the ovaries, it can create fluid-filled abscesses that function similarly to cysts. Unlike functional cysts, infection-related cysts require antibiotic treatment and sometimes drainage.
When Cysts Become a Problem
Most ovarian cysts cause no symptoms and vanish without anyone knowing they existed. Problems arise mainly in two scenarios: rupture and torsion.
A ruptured cyst spills its fluid into the pelvic cavity, which can cause sudden, sharp pain on one side of the lower abdomen. Corpus luteum cysts are more likely to rupture than follicular cysts, and endometriomas can cause particularly intense pain when they leak their thick, inflammatory contents. Most ruptures resolve with rest and pain management, but heavy internal bleeding occasionally requires emergency care.
Torsion happens when a cyst makes the ovary heavy or unstable enough to twist on its own blood supply. The risk increases significantly once a cyst exceeds 5 centimeters in diameter. Torsion cuts off blood flow to the ovary and causes severe, sudden pain, often with nausea. It requires urgent treatment to save the ovary.
Size alone isn’t always a reliable indicator of danger. A 3-centimeter functional cyst during a normal cycle is unremarkable. A 3-centimeter endometrioma tells a different story. The type of cyst, its contents, and whether it’s changing over time all matter more than diameter alone.