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 shrink back down afterward. These “functional” cysts are the most common type and typically resolve on their own within two to three menstrual cycles. Other cysts develop from underlying conditions like endometriosis, hormonal disorders, infections, or fertility treatments.
Functional Cysts: The Most Common Type
Every month, your ovary grows a small fluid-filled sac called a follicle. Inside it, an egg matures. At mid-cycle, a surge of hormones triggers the follicle to open and release the egg. A functional cyst forms when something in this process goes off-script.
There are two main varieties. A follicular cyst develops when the follicle doesn’t rupture and release its egg. Instead, it keeps growing, typically reaching over 2.5 cm in diameter, and can cause a feeling of heaviness or discomfort on one side. The other type, a corpus luteum cyst, happens in the second half of the cycle. After the egg is released, the empty follicle normally shrinks into a small structure called the corpus luteum, which produces progesterone for about 14 days before dissolving. If it doesn’t dissolve and instead fills with fluid, it becomes a corpus luteum cyst, generally defined as 3 cm or larger.
Both types are driven by hormonal signaling. A follicular cyst can result from too much follicle-stimulating hormone (FSH) or from missing the mid-cycle surge of luteinizing hormone (LH) that triggers ovulation. These hormonal misfires are common, which is why most people with ovaries will develop a functional cyst at some point without ever knowing it.
Corpus Luteum Cysts During Pregnancy
If you become pregnant, your corpus luteum doesn’t dissolve on its usual 14-day schedule. It sticks around to produce the progesterone your uterus needs to sustain the early pregnancy. It keeps doing this job until the placenta takes over, usually around week 12. During that time, the corpus luteum can fill with fluid and expand into a cyst. These cysts are harmless and almost always disappear on their own during the second trimester. They’re often discovered incidentally during a first-trimester ultrasound.
Hormonal Conditions Like PCOS
Polycystic ovary syndrome (PCOS) causes a distinct pattern of cyst formation. Rather than one large cyst, the ovaries develop many small fluid-filled sacs clustered along their outer edges. Each sac contains an immature egg that never fully developed or was released. The ovaries may also be slightly larger than usual.
The driving force is a hormonal imbalance involving several interconnected systems. Higher-than-normal levels of androgens (often called “male hormones,” though all bodies produce them) prevent the ovaries from releasing eggs. Insulin resistance plays a role too: elevated insulin stimulates the ovaries to produce more androgens, which further suppresses ovulation. The result is a cycle of unreleased eggs that accumulate as small cysts. Notably, the large singular cysts that send people to the emergency room are not a feature of PCOS.
Endometriomas From Endometriosis
Endometriomas, sometimes called chocolate cysts because of their dark brown contents, form when tissue similar to the uterine lining grows on or inside an ovary. The leading theory for how this happens involves retrograde menstruation: during your period, some menstrual tissue flows backward through the fallopian tubes instead of leaving the body through the vagina. When that tissue lands on an ovary, it responds to hormonal cycles just like the uterine lining does, thickening and bleeding with each period. Over time, the repeated bleeding and inflammation create a walled-off cyst filled with old blood.
Endometriomas tend to be more persistent than functional cysts. They don’t typically resolve on their own and can grow over multiple cycles, sometimes causing significant pelvic pain, especially during menstruation.
Pelvic Infections
Pelvic inflammatory disease (PID) can lead to a specific type of cyst-like mass called a tubo-ovarian abscess. PID is an infection of the upper reproductive tract, often caused by sexually transmitted bacteria such as gonorrhea or chlamydia, though normal vaginal bacteria can also be involved. When infection spreads from the cervix up through the uterus and into the fallopian tubes, it can reach the ovaries. The resulting inflammation and infection can form an abscess, a pocket of infected fluid that involves the tube and ovary together. These are not cysts in the traditional sense, but they can look similar on imaging and cause severe pelvic pain, fever, and tenderness.
Fertility Medications
Fertility drugs that stimulate ovulation significantly increase the chance of developing ovarian cysts. These medications work by pushing the ovaries to produce more follicles than they normally would in a single cycle. In clinical trials of one widely used ovulation-stimulating medication, ovarian enlargement occurred in about 13.6% of patients. In some cases, the ovaries respond too aggressively, a condition called ovarian hyperstimulation syndrome, which can cause rapid, significant ovarian swelling within 24 hours to several days.
Because of this risk, doctors check for existing cysts before starting each treatment cycle. If enlargement occurs, treatment is paused until the ovaries return to normal size. The good news is that cysts caused by fertility drugs usually shrink on their own within a few days to weeks after the medication is stopped.
What Determines Whether a Cyst Needs Treatment
Size is the single biggest factor in how a cyst gets managed. In premenopausal people, simple cysts smaller than 5 cm don’t need follow-up at all. Cysts between 5 and 7 cm are monitored with ultrasound every three to six months to see if they’re growing or shrinking. Cysts larger than 7 cm are typically considered for surgical removal because of the risk of ovarian torsion, where the weight of the cyst causes the ovary to twist on itself, cutting off its blood supply.
For postmenopausal people, the thresholds are more conservative. Simple cysts under 5 cm with normal blood markers carry a low risk of malignancy and are monitored with regular ultrasounds. Anything larger than 5 cm, or any cyst with a complex appearance (thick walls, internal divisions, solid areas), gets referred for further evaluation.
Most functional cysts in premenopausal people resolve within two to three menstrual cycles without any intervention. The discomfort they cause, a dull ache or pressure on one side, usually fades as the cyst shrinks. Cysts from endometriosis, PCOS, or infections follow different timelines and generally require condition-specific treatment rather than watchful waiting.