Most ovarian cysts form as a normal part of your menstrual cycle, when a fluid-filled sac either fails to release an egg or doesn’t shrink back down after releasing one. About 20% of women develop at least one ovarian mass during their lifetime, and the vast majority are harmless. But cysts can also develop from endometriosis, infections, or cells left over from embryonic development. Understanding which type you’re dealing with helps explain why it showed up and what, if anything, needs to happen next.
Functional Cysts: The Most Common Type
Every month, one of your ovaries grows a small fluid-filled sac called a follicle. Inside that follicle, an egg matures. When the egg is ready, the follicle ruptures, releases the egg, and the leftover structure (called the corpus luteum) gradually breaks down. A functional cyst forms when something in that routine goes slightly off track. There are two versions.
A follicular cyst happens when the follicle never ruptures. Instead of releasing the egg, it keeps growing and fills with fluid. These cysts are usually painless and resolve on their own within one to three menstrual cycles.
A corpus luteum cyst happens after the egg has already been released. Normally, the empty follicle shrinks and dissolves. But sometimes the opening where the egg came out seals shut, trapping fluid inside. The sac swells into a cyst that can grow several centimeters wide. Corpus luteum cysts occasionally cause a dull ache on one side of the pelvis, but they also tend to disappear without treatment within a few weeks.
Both types are considered a byproduct of normal ovulation. If you’re having regular periods, your ovaries are producing these follicles every month, which means functional cysts can show up repeatedly throughout your reproductive years.
How PCOS Causes Multiple Small Cysts
Polycystic ovary syndrome works differently from a single functional cyst. In PCOS, a hormonal imbalance prevents follicles from maturing fully. Elevated levels of a hormone called luteinizing hormone drive the ovaries to produce too many androgens (sometimes called “male hormones,” though all women produce them in smaller amounts). At the same time, there isn’t enough follicle-stimulating hormone to push any single follicle to the finish line.
The result is that multiple follicles start developing but stall partway through. They line up along the outer edge of the ovary, visible on ultrasound as a string of small cysts. None of them release an egg, which is why people with PCOS often have irregular or absent periods.
Insulin resistance makes this worse. High insulin levels act on the ovaries like an accelerator for androgen production, creating a cycle where excess androgens block ovulation, which keeps more immature follicles accumulating. This is why weight management and medications that improve insulin sensitivity are often part of PCOS treatment: they address the upstream cause rather than the cysts themselves.
Endometriomas (Chocolate Cysts)
If you have endometriosis, tissue similar to the lining of the uterus can grow in places it shouldn’t, including on or inside your ovaries. One leading theory is that during menstruation, some of this 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 your hormonal cycle the same way it would inside the uterus: it thickens, breaks down, and bleeds each month.
Because the blood has nowhere to go, it collects inside a cyst. Over many cycles the trapped blood turns dark brown, which is why these are sometimes called chocolate cysts. Endometriomas can cause significant pelvic pain, especially during periods, and may affect fertility if they grow large enough to damage surrounding ovarian tissue. Unlike functional cysts, they rarely resolve on their own.
Dermoid Cysts: Present From Birth
Dermoid cysts are among the strangest things your body can produce. They form from germ cells, the same cells that eventually become eggs. During fetal development, germ cells contain three layers that are supposed to turn into different types of tissue: one layer becomes skin and hair, another becomes muscle and connective tissue, and a third becomes internal organs. Sometimes, for reasons scientists still don’t fully understand, some of these cells develop into mature tissue inside the ovary.
This means a dermoid cyst can contain fully formed skin, hair, teeth, nerve tissue, or even small amounts of brain tissue, all bundled together in a sac. They form before you’re born but may not be discovered until adulthood, when they’ve grown large enough to cause symptoms or show up on an imaging scan done for another reason. Dermoid cysts are almost always benign, but they don’t go away on their own and are typically removed surgically if they become large or painful.
Cysts From Pelvic Infections
Pelvic inflammatory disease, usually caused by sexually transmitted bacteria like gonorrhea or chlamydia, can also lead to cyst formation. The bacteria travel from the cervix into the uterus and sometimes reach the ovaries, where they can trigger the development of infected cysts or abscesses.
These cysts are filled with bacteria rather than the clear fluid found in functional cysts. If an infected cyst ruptures, the bacteria can spill into the abdominal cavity and potentially cause sepsis, a serious and sometimes life-threatening blood infection. Pelvic infections that are caught and treated early with antibiotics are far less likely to progress to this point.
Cysts During Pregnancy
A corpus luteum cyst can form in early pregnancy for the same reason it forms during a regular cycle: the corpus luteum seals shut and fills with fluid. During pregnancy, though, the corpus luteum has an important job. It produces progesterone and estrogen to support the pregnancy until the placenta takes over, usually around 10 to 12 weeks. A cyst that develops during this process is generally monitored but left alone, since it typically shrinks once the placenta is fully functional.
Who Is Most at Risk
Several factors raise the likelihood of developing ovarian cysts. Ovulation itself is the biggest one, so anyone who menstruates regularly is at baseline risk for functional cysts. Fertility medications that stimulate ovulation can increase the chances further by encouraging multiple follicles to develop at once.
Endometriosis raises your risk for endometriomas specifically. A history of pelvic inflammatory disease makes infection-related cysts more likely. And hormonal imbalances like those seen in PCOS create the conditions for multiple small cysts to accumulate over time.
Hormonal birth control that suppresses ovulation actually reduces the risk of new functional cysts forming, since it prevents the monthly follicle development that gives rise to them in the first place.
When a Cyst Is Worth Investigating
In premenopausal women, the odds of a cyst being cancerous are roughly 1 in 1,000. That number rises to about 3 in 1,000 by age 50. The overwhelming majority of ovarian cysts are benign and many disappear without any intervention.
Cysts that are large (generally over 5 to 7 centimeters), solid rather than fluid-filled, or persistent across multiple menstrual cycles are more likely to be evaluated with follow-up ultrasound or blood work. Sudden, sharp pelvic pain can signal a ruptured cyst or ovarian torsion, where a large cyst causes the ovary to twist on itself, cutting off blood flow. Both situations need prompt medical attention, but they’re the exception rather than the rule. Most cysts are discovered incidentally, watched briefly, and never cause a problem.