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 the way it should afterward. These “functional” cysts are by far the most common type and usually resolve on their own within a few weeks. Other cysts develop from abnormal cell growth, endometriosis, infections, or hormonal conditions, and those tend to need closer monitoring.
How Your Menstrual Cycle Creates Cysts
To understand the most common ovarian cysts, it helps to know what your ovaries do each month. During the first half of your cycle, a hormone called FSH tells your ovary to grow a small fluid-filled sac called a follicle. Inside that follicle, an egg matures. Around mid-cycle, a surge of another hormone, LH, triggers the follicle to open and release the egg. After ovulation, the leftover follicle wall transforms into a temporary structure called the corpus luteum, which produces progesterone to support a possible pregnancy. If pregnancy doesn’t happen, the corpus luteum breaks down within about 14 days, and the cycle starts over.
Problems at either stage can produce a cyst:
- Follicular cysts form when the follicle never ruptures to release the egg. Instead, it keeps growing under hormonal stimulation. These cysts are typically larger than 2.5 cm and can cause a feeling of heaviness or discomfort on one side of the pelvis. They usually shrink and disappear within one to three menstrual cycles.
- Corpus luteum cysts form after the egg has been released, when the corpus luteum seals shut and fills with fluid or blood instead of breaking down. A small blood vessel on the ovary’s surface can get disrupted during ovulation, causing the cyst to fill with blood. These cysts can occasionally rupture, which may cause sudden, sharp pain.
Both types are considered functional cysts. They’re tied directly to the normal hormonal rhythm of your cycle and are rarely dangerous.
PCOS and Insulin Resistance
Polycystic ovary syndrome is one of the most well-known causes of persistent ovarian cysts. In PCOS, the hormonal signals that guide egg development and ovulation are disrupted. Multiple follicles start growing each cycle, but none of them mature fully or release an egg. These stalled follicles remain on the ovaries as small cysts, giving the ovaries their characteristic “string of pearls” appearance on ultrasound.
Insulin resistance plays a central role. When your cells stop responding normally to insulin, your body compensates by producing more of it. Elevated insulin levels interfere with ovulation and stimulate the ovaries to produce excess androgens (hormones like testosterone). That combination keeps follicles from completing their development. PCOS affects an estimated 6 to 12 percent of women of reproductive age, making it one of the most common hormonal disorders tied to cyst formation.
Endometriomas (Chocolate Cysts)
Endometriosis occurs when tissue similar to the uterine lining grows outside the uterus. When that tissue attaches to an ovary, it can form a cyst called an endometrioma. The leading theory is that during menstruation, some tissue flows backward through the fallopian tubes and onto the ovaries instead of leaving the body. Once there, this tissue thickens and bleeds with each menstrual cycle, just like the lining inside the uterus. Over time, the repeated bleeding and inflammation creates a cyst filled with old, dark blood, which is why these are sometimes called chocolate cysts.
Endometriomas don’t resolve on their own the way functional cysts do. They tend to grow over time and can cause significant pelvic pain, painful periods, and fertility problems.
Dermoid Cysts and Cystadenomas
Some ovarian cysts aren’t related to your menstrual cycle at all. They grow from different cell types within the ovary.
Dermoid cysts develop from germ cells, the cells that would normally become eggs. These germ cells contain three layers that, during fetal development, give rise to skin, hair, muscle, connective tissue, and internal organs. Because of this, dermoid cysts can contain surprisingly varied tissue: hair, teeth, skin, and even sweat glands. They grow slowly, often over years, and are most commonly found in women during their reproductive years. While their contents sound alarming, the vast majority are benign.
Cystadenomas develop from the cells on the outer surface of the ovary. They fill with watery or mucus-like fluid and can grow quite large, sometimes reaching 12 inches or more in diameter. Like dermoid cysts, they don’t typically go away on their own and may need to be removed if they cause symptoms or continue growing.
Fertility Medications
Drugs used to stimulate ovulation can increase your chances of developing ovarian cysts. Clomiphene citrate, one of the most commonly prescribed fertility medications, works by pushing the ovaries to produce and release eggs. In a study published in Fertility and Sterility, only about 1.7% of patients had an ovarian cyst before their first cycle of clomiphene. By subsequent treatment cycles, that number jumped to 21.5%. The medication’s stimulating effect on the ovaries essentially creates the conditions for follicular cysts to form, particularly when multiple follicles are recruited but not all of them release their eggs.
Infections and Abscesses
Pelvic inflammatory disease, a bacterial infection of the upper reproductive tract, can lead to fluid-filled pockets of infection on the ovaries called tubo-ovarian abscesses. These aren’t cysts in the traditional sense, but they appear as cyst-like masses on imaging and can be mistaken for other types of ovarian cysts. PID is usually caused by sexually transmitted bacteria that spread from the cervix into the uterus, fallopian tubes, and ovaries. Left untreated, these abscesses can become life-threatening. They’re accompanied by fever, severe pelvic pain, and abnormal discharge, which helps distinguish them from other cyst types.
Cysts During Pregnancy
Corpus luteum cysts are common in early pregnancy. After conception, the corpus luteum doesn’t break down on its usual 14-day schedule. Instead, it keeps producing progesterone to sustain the pregnancy until the placenta takes over, usually around 10 to 12 weeks. Sometimes the corpus luteum swells with fluid during this period and forms a cyst. These pregnancy-related cysts are almost always harmless and shrink on their own during the second trimester.
When Size and Features Matter
Most ovarian cysts are small, painless, and discovered incidentally during an ultrasound for something else. What determines whether a cyst needs attention is a combination of its size, structure, and whether you’re still having periods.
In postmenopausal women, guidelines from the Royal College of Obstetricians and Gynaecologists classify simple, single-chamber cysts of 3 cm or smaller as low risk, requiring no follow-up. Cysts between 3 and 5 cm with normal blood markers can be monitored with a repeat ultrasound in four to six months. Cysts larger than 5 cm, or those that are complex (multiple chambers, solid areas, or present on both ovaries), are more likely to be evaluated surgically.
In premenopausal women, the threshold for concern is generally higher because functional cysts are so common. A simple cyst under 5 to 7 cm in a younger woman is usually watched rather than treated. The presence of pain, rapid growth, or complex features on ultrasound shifts the approach toward further evaluation.
Risk Factors That Increase Your Chances
Several factors make ovarian cysts more likely. Having had a cyst before raises your odds of developing another. Irregular menstrual cycles, particularly those associated with PCOS, mean your ovaries are more frequently failing to complete the ovulation process. Early onset of menstruation and obesity (through its link to insulin resistance) also contribute. Hypothyroidism has been loosely associated with changes in ovarian size and cyst formation, though current evidence does not support a direct causal link between thyroid disease and ovarian cysts or PCOS.
Hormonal contraceptives that suppress ovulation reduce the likelihood of new functional cysts forming, which is why they’re sometimes recommended for women who develop recurrent cysts. They don’t shrink existing cysts, but they can prevent the hormonal cycle that creates new ones.