Women get cysts primarily because of the normal hormonal cycle their bodies run every month. Each menstrual cycle involves growing, filling, and rupturing tiny fluid-filled sacs in the ovaries, and any disruption in that process can leave a cyst behind. Most cysts are harmless, resolve on their own, and never cause symptoms. Up to 10% of women will have surgery for an ovarian cyst at some point in their lives, but the vast majority of cysts found in premenopausal women are benign, with only about 1 to 3 in 1,000 turning out to be cancerous.
The Monthly Cycle That Makes Cysts So Common
Every month, one of your ovaries grows a small fluid-filled sac called a follicle. That follicle produces estrogen and progesterone, and around the middle of your cycle, a surge of hormones causes the follicle wall to rupture and release an egg. After the egg leaves, the leftover follicle tissue (called the corpus luteum) keeps producing progesterone for about 14 days to support a potential pregnancy. If pregnancy doesn’t happen, the corpus luteum dissolves and the cycle starts over.
This process creates two common opportunities for a cyst to form:
- Follicular cysts happen when the follicle never ruptures. Instead of releasing the egg, it keeps growing and filling with fluid. This can result from insufficient hormone signaling at mid-cycle, particularly when the usual surge of luteinizing hormone doesn’t occur on schedule.
- Corpus luteum cysts form after the egg is released. Sometimes the opening where the egg escaped seals shut, trapping fluid inside the remaining tissue. These cysts typically grow to about 3 centimeters and usually resolve without treatment.
Both of these are called functional cysts because they arise from the ovary doing its job. They’re the most common type, and most are found incidentally on imaging done for other reasons, meaning the woman never knew the cyst was there.
How PCOS Creates a Different Kind of Cyst
Polycystic ovary syndrome involves a hormonal imbalance that stalls follicle development partway through. In a typical cycle, one dominant follicle matures fully and releases an egg. In PCOS, excess androgens (often called “male hormones,” though all women produce them) combined with elevated insulin and insufficient follicle-stimulating hormone create an environment where multiple follicles start growing but get stuck at 5 to 8 millimeters. None of them mature enough to ovulate, so they accumulate along the ovary’s surface, giving it the characteristic “string of pearls” appearance on ultrasound.
This isn’t a one-time event. Because ovulation doesn’t happen reliably, the cycle of arrested follicles repeats month after month. The underlying drivers are persistent high androgen levels and disrupted signaling between the brain’s pituitary gland and the ovaries. That’s why PCOS is treated as a hormonal condition rather than simply an ovarian one.
Endometriomas: Cysts From Misplaced Tissue
Endometriomas, sometimes called “chocolate cysts” because of their dark, thick contents, form when tissue similar to the uterine lining grows on or inside the ovary. The leading explanation is that during menstruation, some blood and endometrial fragments travel backward through the fallopian tubes into the pelvic cavity. This retrograde menstruation is actually extremely common: blood can be found in the pelvic fluid of about 90% of menstruating women. So backward flow alone isn’t enough to cause endometriosis.
What seems to matter is whether those fragments arrive intact, with their cellular architecture preserved, and whether the immune system clears them or allows them to implant. When fragments with both glandular and supportive tissue attach to the ovary, they respond to monthly hormonal shifts just like normal uterine lining would. They bleed with each cycle, but the blood has nowhere to go. Over time, it pools inside the ovary and forms a cyst filled with old, oxidized blood.
Dermoid Cysts: Present From Before Birth
Dermoid cysts are one of the stranger things a body can produce. They form from germ cells, the same cells that eventually become eggs. During fetal development, germ cells contain three foundational layers that normally differentiate into every tissue in the body: one layer becomes skin, hair, and teeth; another becomes muscle and connective tissue; the third becomes internal organs and gut lining. Sometimes these layers grow abnormally, producing mature tissue like fully formed hair, teeth, skin, and even nerve or brain tissue, all bundled together inside a sac on the ovary.
Because they originate during fetal development, dermoid cysts are technically present from before birth, though they may not be discovered until adulthood. They grow slowly and are almost always benign.
Breast Cysts and Hormonal Fluctuations
Cysts don’t only form on the ovaries. Breast cysts are part of a broader pattern called fibrocystic changes, which show up in about 50% of women examined clinically and up to 90% when breast tissue is examined under a microscope. Estrogen drives the process: it stimulates cell growth and proliferation in breast tissue while also causing the connective tissue between milk-producing lobules to swell with fluid. Over time, these fluid-filled pockets can enlarge into palpable cysts. They tend to fluctuate with the menstrual cycle, becoming more noticeable or tender in the days before a period when estrogen levels are shifting.
Bartholin’s Cysts: A Blocked Gland
The Bartholin’s glands sit on each side of the vaginal opening and produce fluid that helps with lubrication. When the tiny duct that drains one of these glands gets blocked, whether from minor injury, irritation, or infection, fluid backs up into the gland and forms a cyst. These cysts are usually painless unless they become infected, at which point they can develop into an abscess. They’re unrelated to the hormonal mechanisms behind ovarian or breast cysts and instead result from simple plumbing: a blocked exit for fluid that keeps being produced.
Why Hormones Are the Common Thread
The pattern across most cyst types is hormonal activity. Functional ovarian cysts form because of the monthly hormone-driven cycle of follicle growth and ovulation. PCOS cysts result from a hormonal environment that stalls that cycle. Endometriomas grow in response to the same cyclical hormones that control the uterine lining. Breast cysts develop under estrogen’s influence on breast tissue. This is why cysts are far more common during the reproductive years and tend to become less frequent after menopause, when estrogen and progesterone levels drop significantly.
Certain medications and treatments can also shift the balance. Fertility drugs that stimulate the ovaries to produce multiple follicles can increase cyst risk, and the large follicular cysts that result from ovulation induction carry a higher chance of a serious complication called ovarian torsion, where the ovary twists on its own blood supply.
When Size Starts to Matter
Most cysts are small and cause no symptoms. Problems tend to arise with size. Cysts larger than 5 centimeters are the primary risk factor for ovarian torsion, with more than 80% of torsion cases involving a mass of that size or larger. Torsion has been reported with cysts ranging from 1 to 30 centimeters, with an average of about 9.5 centimeters. It occurs more often with benign masses than cancerous ones, likely because benign cysts are far more common and tend to be mobile.
Rupture is the other concern. A cyst that bursts can cause sudden, sharp pain on one side of the pelvis. For most functional cysts, rupture is brief and self-limiting. For endometriomas or larger cysts, rupture can cause more significant pain and sometimes internal bleeding that requires medical attention. Persistent or worsening pelvic pain, sudden severe pain, or pain accompanied by fever or dizziness are signs that something beyond a routine cyst may be happening.