Most ovarian cysts form as a normal part of the menstrual cycle when a follicle either doesn’t release its egg or doesn’t shrink back down after ovulation. These “functional” cysts are by far the most common type and usually resolve on their own within a few weeks. But ovarian cysts can also develop from hormonal imbalances, abnormal cell growth, endometriosis, infections, fertility treatments, and pregnancy. The cause determines whether a cyst is harmless or needs attention.
Functional Cysts and the Normal Menstrual Cycle
Every month, your ovary grows a small fluid-filled sac called a follicle, which holds a maturing egg. When everything works correctly, the follicle ruptures to release the egg, then collapses into a temporary structure called the corpus luteum that produces hormones to support a potential pregnancy. Functional cysts happen when this process stalls at one of two points.
A follicular cyst forms when the follicle doesn’t rupture. Instead of releasing the egg, the sac keeps growing and fills with fluid. These cysts are typically painless and disappear within one to three menstrual cycles.
A corpus luteum cyst forms after ovulation, when the collapsed follicle seals itself off and fluid accumulates inside. Research shows that about 78% of corpora lutea naturally develop a central fluid-filled cavity in the days after ovulation, a completely normal phenomenon. In most women this cavity shrinks and disappears. But occasionally it persists, grows larger, or bleeds internally, creating a cyst that can cause a dull ache on one side of the pelvis. Corpus luteum cysts usually resolve on their own too, though ones that bleed may cause sharper pain.
Hormonal Imbalances and PCOS
Polycystic ovary syndrome (PCOS) is one of the most well-known hormonal causes of ovarian cysts. Despite the name, the “cysts” in PCOS are actually many small, immature follicles that never grow large enough to ovulate. They accumulate along the outer edge of the ovary, giving it a characteristic appearance on ultrasound.
The underlying problem is a reinforcing loop between excess androgens (often called “male hormones,” though all women produce them) and high insulin levels. Excess androgens speed up the early recruitment of follicles but then block the final step where one dominant follicle is selected and released. At the same time, high insulin levels make the ovary more sensitive to hormonal signals that drive androgen production, which keeps the cycle going. The result is a relative drop in the hormone that normally promotes follicle maturation (FSH) and a rise in the hormone that stimulates androgen output (LH). Follicles start developing but stall partway, lining up as small fluid-filled sacs that never complete ovulation.
Insulin resistance plays a central role here, which is why PCOS is more common in people with higher body weight, though it can affect anyone regardless of size.
Endometriosis and Chocolate Cysts
Endometriosis occurs when tissue similar to the uterine lining grows in places it shouldn’t, including on or inside the ovaries. When this tissue implants on the ovary, it responds to monthly hormonal changes the same way the uterine lining does: it thickens, breaks down, and bleeds. But the blood has nowhere to go. Over time, it collects inside a cyst filled with dark, old blood, which is why surgeons call these “chocolate cysts” (the medical term is endometrioma).
Endometriomas tend to recur. Studies tracking women after surgical removal report recurrence rates between 10% and 35%. Women who don’t receive hormonal treatment afterward have roughly three times the risk of recurrence compared to those who do. Pregnancy after surgery also appears protective, reducing recurrence by a similar margin.
Dermoid Cysts and Abnormal Cell Growth
Dermoid cysts are one of the stranger things the human body can produce. They form from germ cells, the precursors to egg cells, which carry the genetic blueprint for building any type of tissue. Sometimes these cells begin developing on their own, producing fully formed skin, hair, teeth, sweat glands, nerve tissue, and even brain tissue inside a cyst wall. The result is a slow-growing mass that can contain a bizarre mix of body parts.
Dermoid cysts are present from birth, though they may not be discovered until adulthood. They’re almost always benign. The main concern is size: cysts larger than 5 to 6 centimeters carry a higher risk of causing the ovary to twist on itself (ovarian torsion), which cuts off blood supply and requires emergency surgery. Vomiting alongside sudden pelvic pain is a strong indicator of torsion, with about 70% sensitivity and 80% specificity for that complication.
Fertility Medications
Drugs used to stimulate ovulation can cause cysts as a direct side effect. Clomiphene citrate, one of the most commonly prescribed fertility medications, works by tricking the brain into releasing more of the hormones that drive follicle growth. This can cause multiple follicles to develop simultaneously. In clinical trials involving over 8,000 women, 13.6% experienced ovarian enlargement while taking the drug.
In more severe cases, the ovaries can overrespond, a condition called ovarian hyperstimulation syndrome (OHSS). The ovaries swell dramatically, and fluid can leak into the abdomen, causing bloating, nausea, vomiting, and rapid weight gain. OHSS can progress within 24 hours from mild discomfort to a serious medical situation. Women with PCOS are particularly susceptible because their ovaries are already primed to overreact to hormonal stimulation.
Pregnancy-Related Cysts
During early pregnancy, the corpus luteum normally persists to produce progesterone until the placenta takes over, usually around 10 to 12 weeks. This sustained corpus luteum sometimes grows into a cyst. It’s usually harmless and resolves as the pregnancy progresses.
A less common type, called theca lutein cysts, can develop when the ovary becomes hypersensitive to the pregnancy hormone hCG. These tend to appear in situations where hCG levels are unusually high, such as twin pregnancies or certain pregnancy complications. They’re typically found incidentally on ultrasound and resolve after delivery.
Pelvic Infections
Bacterial infections that start in the lower reproductive tract can climb upward through the cervix and uterus into the fallopian tubes and ovaries. This progression, known as pelvic inflammatory disease (PID), damages the tissue lining these structures. If the infection reaches the ovary, it can form a pus-filled mass called a tubo-ovarian abscess, which involves both the fallopian tube and the ovary fused together by inflammation.
This type of cyst-like mass is distinct from the others on this list because it’s driven by active infection rather than hormonal or developmental processes. It primarily affects sexually active women of reproductive age and is most often a consequence of untreated PID. Unlike functional cysts, tubo-ovarian abscesses don’t resolve on their own and require treatment to clear the infection.
When Size and Appearance Matter
Most ovarian cysts are discovered incidentally during imaging for something else, and the majority need no treatment. What determines the next step is a combination of size, internal appearance, and symptoms. Simple fluid-filled cysts under 5 centimeters are almost always benign and are typically monitored with a follow-up ultrasound. Cysts larger than 5 to 6 centimeters raise concern for torsion. Those over 10 centimeters generally warrant surgical removal, and cysts exceeding 12 centimeters are more likely to require a larger incision rather than minimally invasive surgery.
Internal features matter as much as size. A cyst that’s entirely filled with clear fluid looks very different on ultrasound from one containing solid areas, thick walls, or mixed tissue. Dermoid cysts, for example, produce distinctive patterns because of the hair, fat, and calcified material inside them. Cysts with complex features are evaluated more carefully because, in rare cases, they can indicate borderline or malignant growths, particularly in women over 50.