Most ovarian cysts form as a normal part of your menstrual cycle, and you don’t need to do anything specific to “get” one. In fact, the majority of people with ovaries will develop at least one cyst during their reproductive years without ever knowing it. Cysts form when the regular process of releasing an egg doesn’t go quite as expected, or when certain health conditions, medications, or lifestyle factors shift the odds. Understanding what causes them can help you make sense of a diagnosis or recognize why you might be more prone to them.
How Normal Ovulation Creates Cysts
Every month, your ovary grows a small fluid-filled sac called a follicle. Inside it, an egg matures. When the egg is ready, the follicle ruptures, releasing the egg into the fallopian tube. After that, the empty follicle transforms into a temporary structure called the corpus luteum, which produces progesterone to support a potential pregnancy. If pregnancy doesn’t happen, the corpus luteum breaks down after about 14 days and your period starts.
Two things can go wrong in this otherwise routine process, and both result in what doctors call functional cysts:
- Follicular cysts form when the follicle never ruptures. Instead of releasing the egg, it keeps growing under hormonal stimulation. These cysts tend to be smooth, thin-walled, and filled with fluid.
- Corpus luteum cysts form when the empty follicle doesn’t dissolve on schedule. Fluid or blood accumulates inside it, and it typically grows to around 3 cm. During early pregnancy, the corpus luteum normally sticks around to produce progesterone, remaining stable in size through about 9 weeks of gestation before shrinking or disappearing between weeks 10 and 13. A corpus luteum cyst develops when this structure persists longer than it should.
Functional cysts are the most common type. They usually resolve on their own within one to three menstrual cycles without any treatment.
Hormonal Imbalances and PCOS
The two hormones that orchestrate ovulation, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), need to rise and fall in a specific pattern for the follicle to rupture properly. When FSH stimulation is too high or the mid-cycle LH surge doesn’t happen, the follicle can stall and become a cyst.
Polycystic ovary syndrome (PCOS) is one of the most well-known examples of this kind of hormonal disruption. In PCOS, multiple follicles begin developing each cycle but none of them fully mature or release an egg. The result is a cluster of small cysts visible on ultrasound, along with irregular periods and other hormonal symptoms. If you’ve been told you have PCOS, the cysts themselves aren’t dangerous, but they reflect an ongoing pattern of incomplete ovulation that can affect fertility and overall health.
Fertility Medications
Drugs that stimulate ovulation are a recognized trigger for ovarian cysts. Clomiphene citrate, one of the most commonly prescribed fertility medications, works by pushing the ovaries to develop and release eggs. A known side effect is ovarian cyst formation and ovarian enlargement. Lowering the dose or shortening the treatment duration reduces this risk. If you’re undergoing fertility treatment and develop pelvic pressure or bloating, a cyst from ovarian stimulation is a likely explanation.
Endometriosis
Endometriosis occurs when tissue similar to the uterine lining grows outside the uterus. When this tissue attaches to an ovary, it can form a type of cyst called an endometrioma, sometimes referred to as a “chocolate cyst” because of the dark, old blood inside it. Unlike functional cysts, endometriomas don’t resolve on their own with your menstrual cycle. They tend to grow over time and can cause significant pain, especially during periods or intercourse. Having endometriosis is an independent risk factor for developing ovarian cysts.
Smoking and Body Weight
Cigarette smoking increases the risk of functional ovarian cysts, and body weight plays an interesting role in how much. A population-based study of 586 cyst cases and 757 controls found that current smokers with a BMI under 20 had roughly 2.5 times the risk of developing a functional cyst compared to nonsmokers. For smokers with a BMI between 20 and 25, the risk was about 1.6 times higher. But for smokers with a higher BMI, the increased risk essentially disappeared. In other words, being underweight or at the lower end of normal weight seems to amplify smoking’s effect on cyst formation. The exact mechanism isn’t fully understood, but it likely involves how nicotine and body fat both influence the hormones that control ovulation.
Other Types of Ovarian Cysts
Not all cysts come from ovulation gone sideways. Some form for entirely different biological reasons:
- Dermoid cysts (teratomas) develop from the cells that produce eggs. Because those cells have the potential to become any type of tissue, dermoid cysts can contain hair, skin, teeth, or fat. They’re present from birth and grow slowly over years. They’re not caused by anything you did or didn’t do.
- Cystadenomas grow from cells on the outer surface of the ovary. They’re filled with watery or mucous fluid and can become quite large, sometimes causing the ovary to shift or twist.
These non-functional cysts don’t typically go away on their own and are more likely to need monitoring or removal, depending on their size and characteristics.
Pregnancy and Cyst Formation
Early pregnancy actually relies on a structure that can become a cyst. After ovulation, the corpus luteum produces progesterone to sustain the pregnancy until the placenta takes over, around 10 to 14 weeks in. During this window, the corpus luteum can fill with fluid and become a cyst. Most of these are harmless and shrink as the placenta matures. They’re typically discovered incidentally during a first-trimester ultrasound and rarely need treatment. In uncommon cases, a corpus luteum cyst can rupture or twist the ovary, causing sudden sharp pain that needs medical attention.
Who Gets Cysts Most Often
Ovarian cysts are overwhelmingly common in people who are still menstruating. They become much less frequent after menopause, when ovulation stops. Your risk is higher if you have a history of previous ovarian cysts, use fertility medications, have endometriosis, or smoke (particularly if you’re lean). Severe pelvic infections can also cause cysts by creating inflammation around the ovaries and fallopian tubes.
Most functional cysts never cause symptoms. They form, exist quietly for a few weeks, and dissolve before your next period. The ones that do cause symptoms usually produce a dull ache or pressure on one side of the pelvis, bloating, or pain during certain movements. A cyst that ruptures can cause sudden, sharp pain that fades over hours to days. A cyst that causes the ovary to twist (ovarian torsion) creates intense, unrelenting pain and nausea, and that’s a surgical emergency.
If you’ve been diagnosed with a cyst, it most likely formed because your body did what it does every month, just imperfectly. The factors that raise your risk are largely hormonal, genetic, or related to specific medical conditions rather than anything you can easily control through daily habits.