Ovarian cysts form because of the way your ovaries work each month. Every menstrual cycle, your ovaries grow a small fluid-filled sac to house a developing egg, and most of the time that sac dissolves on its own. When something disrupts this process, the sac sticks around and becomes what doctors call a cyst. The vast majority are harmless, and up to 10% of women will have surgery for an ovarian cyst at some point in their lives, which means the other 90% either never develop a problematic one or have cysts that resolve without intervention.
How Your Normal Cycle Creates Cysts
To understand why cysts form, it helps to know what your ovary does every month. During the first half of your cycle, a hormone called FSH prompts several follicles (tiny sacs containing immature eggs) to start growing. One follicle becomes dominant, and a surge of a second hormone, LH, triggers ovulation, releasing the egg. The leftover shell of that follicle then transforms into a temporary structure called the corpus luteum, which produces progesterone to prepare your uterus for a potential pregnancy. If pregnancy doesn’t happen, the corpus luteum breaks down after about 14 days, your period starts, and the whole process begins again.
Cysts happen when this sequence doesn’t go according to plan, and there are two common ways it can stall.
Follicular Cysts
If the LH surge never happens at midcycle, or if the ovary gets too much FSH stimulation, the dominant follicle doesn’t release its egg. Instead, it keeps growing and fills with fluid. The cells lining the follicle can also persist, pumping out extra estrogen. That excess estrogen sometimes causes heavier or less frequent periods, which is why some women notice cycle changes alongside a cyst.
Corpus Luteum Cysts
After ovulation, the corpus luteum normally shrinks and disappears. Occasionally, though, its exit route seals off and fluid or blood accumulates inside, forming a cyst. These tend to grow larger than follicular cysts and can cause a dull ache on one side. During early pregnancy, the corpus luteum has an especially important job: it produces progesterone until the placenta takes over around week 12. That extended lifespan means corpus luteum cysts are more common in early pregnancy and typically resolve on their own once the placenta matures.
PCOS and Recurring Cysts
Polycystic ovary syndrome is one of the most common reasons women develop cysts repeatedly. The core problem is a hormonal feedback loop. Higher levels of androgens (often called “male hormones,” though all women produce them) prevent your ovaries from releasing eggs. At the same time, many women with PCOS have insulin resistance, meaning their bodies produce more insulin than normal. That extra insulin signals the ovaries to make even more androgens, which further suppresses ovulation.
The result is that month after month, follicles start to develop but never fully mature or release an egg. They accumulate along the outer edge of the ovary, creating the characteristic “string of pearls” appearance on ultrasound. These aren’t dangerous cysts individually, but their presence reflects an ongoing ovulation problem that can affect fertility and overall health.
Endometriomas From Endometriosis
If you have endometriosis, a different type of cyst can form on your ovaries. One leading theory is that during your period, some endometrial-like tissue flows backward through the fallopian tubes instead of leaving the body through the vagina. When this tissue attaches to an ovary, it responds to your hormonal cycle just like the tissue inside your uterus: it thickens, breaks down, and bleeds each month. But with no way to exit, the old blood collects inside a cyst.
Over time, the trapped blood darkens and thickens into a brown, tar-like fluid, which is why these cysts are commonly called “chocolate cysts.” They tend to cause more pain than functional cysts, particularly during periods or sex, and they can affect fertility by damaging surrounding ovarian tissue through ongoing inflammation.
Fertility Treatments and Overstimulation
Fertility medications work by stimulating the ovaries to produce eggs, and sometimes they do the job too well. When the ovaries respond too aggressively, they swell and develop multiple large cysts in a condition called ovarian hyperstimulation syndrome. This is most likely when injectable fertility drugs are combined with a trigger shot of hCG. It rarely happens with oral fertility medications alone.
Mild overstimulation causes bloating and discomfort. In more severe cases, fluid can leak from the swollen ovaries into the abdomen, causing rapid weight gain, nausea, and shortness of breath. If you’re undergoing fertility treatment and notice your abdomen swelling quickly, that warrants a prompt call to your clinic.
Pelvic Infections
Untreated pelvic infections can lead to inflammatory masses on or near the ovaries. Pelvic inflammatory disease, often caused by sexually transmitted bacteria, creates a spectrum of inflammation in the upper reproductive tract. If the infection spreads to the ovary and fallopian tube, it can form a tubo-ovarian abscess: a walled-off pocket of infected fluid that looks and behaves like a complex cyst on imaging. Delayed treatment makes this more likely, which is one reason persistent pelvic pain paired with fever or unusual discharge deserves prompt attention.
When Size Becomes a Concern
Most functional cysts measure 2 to 3 centimeters and resolve within one to three menstrual cycles without you ever knowing they existed. Larger cysts carry more risk. Once a cyst exceeds 5 centimeters, the chance of ovarian torsion increases significantly. Torsion happens when the weight of the cyst causes the ovary to twist on its blood supply, cutting off circulation. This produces sudden, severe pain, often with nausea, and is a surgical emergency.
Rupture is the other complication to be aware of. A cyst can burst during physical activity or sex, releasing its fluid into the pelvic cavity. Small ruptures cause brief, sharp pain that fades. Larger ones, especially those filled with blood, can cause enough internal bleeding to make you feel dizzy or faint. Sharp one-sided pelvic pain that hits suddenly and doesn’t let up within a few hours is worth getting evaluated.
Cancer Risk in Perspective
Many women who discover a cyst immediately worry about cancer. The numbers are reassuring. In premenopausal women, the chance of a symptomatic ovarian cyst being malignant is roughly 1 in 1,000. By age 50, that risk rises to about 3 in 1,000. Doctors use ultrasound features like internal blood flow, solid components, and irregular walls to distinguish suspicious cysts from simple fluid-filled ones. A smooth, thin-walled cyst filled with clear fluid in a younger woman is almost always benign.
What Reduces Your Risk
Hormonal birth control is the most studied option for prevention. By suppressing the hormonal signals that drive follicle development each month, combined oral contraceptives reduce the formation of new functional cysts. There’s also evidence that hormonal contraception helps prevent recurrent cysts in women who keep developing them. It won’t shrink a cyst that already exists, but it can lower the odds of the next one forming.
Beyond medication, there’s no proven lifestyle change that prevents functional cysts specifically. However, managing insulin resistance through regular exercise and a balanced diet can help reduce cyst formation in women with PCOS by lowering the excess insulin that drives androgen production. For endometriomas, hormonal therapies that suppress your menstrual cycle can slow the growth of existing cysts and reduce the chance of new ones.