Most ovarian cysts form as a normal part of your menstrual cycle, when a fluid-filled sac on the ovary either fails to release an egg or doesn’t break down after releasing one. These “functional” cysts are by far the most common type, and the majority resolve on their own within two to three menstrual cycles. Other types develop from embryonic cells, endometrial tissue, or infections, and follow entirely different pathways.
Functional Cysts: The Most Common Kind
Every month, one of your ovaries grows a small fluid-filled sac called a follicle, which houses a developing egg. Around the middle of your cycle, a surge of hormones signals that follicle to rupture and release the egg. A functional cyst forms when some part of this routine process goes off-script. There are two subtypes, and which one you get depends on where in the cycle things stall.
Follicular Cysts
A follicular cyst develops when the follicle never ruptures. Instead of releasing the egg, the sac stays intact and continues to fill with fluid, driven by ongoing hormonal stimulation. These cysts are typically larger than 2.5 centimeters and can cause a dull heaviness or discomfort on one side of the pelvis. The cells lining the follicle may keep producing estrogen, which can throw off your period, making cycles less frequent or unusually heavy.
The usual culprit is a hormonal timing problem: either too much of the hormone that stimulates follicle growth early in the cycle, or a missing surge of the hormone that triggers ovulation at midcycle. Without that trigger, the follicle just keeps growing.
Corpus Luteum Cysts
After a follicle successfully releases an egg, the empty sac normally collapses into a small structure called the corpus luteum, which produces hormones to support a potential pregnancy and then dissolves. A corpus luteum cyst forms when this structure seals shut and fills with fluid instead of breaking down. Sometimes a small blood vessel on the ovary’s surface gets disrupted during ovulation, and the sac fills with blood rather than clear fluid.
Both types of functional cysts are overwhelmingly benign. In premenopausal women, simple cysts under 5 centimeters usually disappear within two to three cycles without any treatment. Cysts between 5 and 7 centimeters are typically monitored with yearly ultrasound. Cysts larger than 7 centimeters generally need advanced imaging or a referral to a gynecologist for possible removal. Even in postmenopausal women, simple cysts up to 10 centimeters on ultrasound are considered likely benign and can often be monitored rather than surgically removed.
Dermoid Cysts
Dermoid cysts are a different category entirely. They don’t come from your menstrual cycle. They form before you’re born, from germ cells, the same cells that eventually become eggs or sperm. During fetal development, these germ cells contain three layers that are supposed to become skin, muscle, organs, and other tissues. Sometimes those layers grow abnormally, with mature tissue bunching together inside a cyst on the ovary.
This is why dermoid cysts can contain strikingly unusual contents: fully formed skin, hair, teeth, sweat glands, nerves, and occasionally even brain tissue. They grow slowly and may not cause symptoms for years, often showing up incidentally on an imaging scan done for another reason. Despite their unusual contents, the vast majority are benign. They do typically require surgical removal because they won’t resolve on their own and can grow large enough to twist the ovary.
Endometriomas
If you have endometriosis, cysts called endometriomas can develop on the ovaries. The leading theory for how this happens involves retrograde menstruation: during your period, some menstrual blood flows backward through the fallopian tubes and into the pelvic cavity instead of exiting the body. Cells from the uterine lining travel with that blood, implant on the ovary, and respond to your monthly hormonal cycle the same way they would inside the uterus. They build up, break down, and bleed, but with no way to exit.
Over repeated cycles of ovulation and retrograde menstruation, this trapped blood accumulates inside a cyst on the ovary. The old, dark blood inside gives them their nickname: “chocolate cysts.” Endometriomas tend to cause significant pain, particularly during periods and sex, and they can affect fertility by damaging healthy ovarian tissue.
What Raises Your Risk
Hormonal imbalances are the single biggest factor. In polycystic ovary syndrome (PCOS), a broader hormonal disruption affecting ovulation and metabolism leads to multiple small follicles developing on the ovaries without maturing or releasing eggs. These appear as a “string of pearls” pattern on ultrasound. This is fundamentally different from a one-off functional cyst, which forms from a single follicle during an otherwise normal cycle. PCOS involves a systemic imbalance, while a functional cyst is a localized, temporary event.
Fertility medications can also increase your risk. Drugs that stimulate ovulation work by pushing your ovaries to develop follicles, and that stimulation can produce unusually large cysts or cause existing cysts to grow. If you already have a cyst when starting a fertility treatment cycle, your doctor will typically want it resolved before proceeding.
Severe pelvic infections, though less common as a cause, can lead to inflammatory masses involving the ovary. Pelvic inflammatory disease, usually caused by sexually transmitted bacteria, can rarely progress to a tubo-ovarian abscess, a serious infection involving the ovary and fallopian tube that requires urgent treatment.
How Hormonal Birth Control Affects Cyst Formation
Combined hormonal contraceptives (the pill, patch, or ring) suppress the hormones that drive follicle growth and ovulation. By keeping those hormone levels low, they prevent the follicular development that leads to functional cysts in the first place. This is why birth control is sometimes prescribed specifically to reduce the recurrence of functional cysts in women who get them repeatedly.
It’s worth noting that hormonal contraceptives prevent new functional cysts from forming, but they don’t shrink cysts that already exist. If you currently have a cyst, going on the pill won’t make it disappear faster. Your body still needs to reabsorb it on its own timeline. The benefit is in prevention, not treatment of an active cyst.
What Cysts Feel Like
Many ovarian cysts cause no symptoms at all and are discovered during a routine pelvic exam or an ultrasound for something unrelated. When they do cause symptoms, the most common is a dull ache or pressure on one side of the lower abdomen, often described as heaviness. Larger cysts can cause bloating, a feeling of fullness, or pain during sex.
Sudden, sharp pelvic pain is a different situation. It can mean a cyst has ruptured, which releases fluid into the pelvic cavity and usually causes intense but short-lived pain. It can also signal ovarian torsion, where a large cyst causes the ovary to twist on its blood supply. Torsion is a surgical emergency because the ovary can lose blood flow. Sudden severe pain on one side, especially with nausea or vomiting, warrants immediate medical attention.