Ovarian cysts grow when hormonal signals go wrong, when abnormal tissue accumulates inside them, or when blood supply feeds their expansion. The specific driver depends entirely on the type of cyst. A normal follicle becomes a cyst when it fails to release an egg or fails to shrink afterward, but endometriomas, dermoid cysts, and cysts linked to polycystic ovary syndrome each enlarge through distinct mechanisms.
How Normal Follicles Become Cysts
Every menstrual cycle, a follicle grows on one of your ovaries and swells to about 2 to 3 centimeters before releasing an egg. This process depends on a precise hormonal chain reaction: rising estrogen signals your brain to release a surge of luteinizing hormone (LH), which triggers ovulation. If that LH surge never comes, the follicle doesn’t rupture. It keeps filling with fluid and becomes a follicular cyst.
The reason the surge fails usually traces back to the hypothalamus, the part of your brain that reads estrogen levels and decides when to trigger ovulation. In some cycles, the hypothalamus becomes temporarily insensitive to estrogen. It stops responding to the signal, so the LH surge is either delayed or never happens. The follicle, still being stimulated by low-level hormones, continues to grow instead of collapsing.
After ovulation, the emptied follicle normally transforms into a small structure called the corpus luteum, which produces progesterone. Progesterone resets the brain’s sensitivity to estrogen, preparing the system for the next cycle. If progesterone doesn’t rise adequately, that reset doesn’t happen, and the next follicle that develops is also more likely to become cystic. This creates a self-reinforcing loop where one missed ovulation can set the stage for another.
Why Cysts Grow During Early Pregnancy
Corpus luteum cysts are common in the first trimester. After conception, the implanting embryo produces rapidly increasing amounts of hCG (human chorionic gonadotropin), the same hormone detected by pregnancy tests. hCG’s job is to keep the corpus luteum alive and producing progesterone, which sustains the pregnancy until the placenta takes over at around 8 to 9 weeks.
Because hCG levels rise exponentially in early pregnancy, the corpus luteum can become overstimulated. It swells with fluid, sometimes reaching several centimeters. These cysts are almost always harmless and shrink on their own once the placenta assumes progesterone production and hCG levels plateau. Occasionally they cause pain if they twist or leak fluid, but they rarely need treatment.
Insulin Resistance and PCOS
In polycystic ovary syndrome, the problem isn’t one large cyst but many small follicles that start developing and then stall. The ovaries end up dotted with follicles that never matured enough to ovulate. Two interconnected forces drive this pattern: excess androgens and insulin resistance.
High insulin levels directly stimulate the ovary’s outer cells to produce more androgens (sometimes called “male hormones,” though all women make them in smaller amounts). Those excess androgens block follicles from maturing. At the same time, insulin resistance promotes higher LH levels from the pituitary gland, which further boosts androgen production and suppresses the FSH needed to push any single follicle to dominance. The result is a cycle where many follicles begin growing but none completes the journey to ovulation, and the partially developed follicles accumulate as small cysts.
Insulin also lowers the liver’s production of sex hormone-binding globulin, a protein that normally soaks up free testosterone in your blood. With less of this protein circulating, more testosterone is active in your body, reinforcing the hormonal imbalance that keeps follicles arrested.
How Endometriomas Expand
Endometriomas, often called chocolate cysts because of their dark, thick contents, grow through a completely different mechanism. They form when tissue similar to the uterine lining implants on or inside the ovary. The leading explanation is retrograde menstruation: during your period, some menstrual tissue flows backward through the fallopian tubes and lands on the ovaries instead of leaving the body.
Once this tissue takes hold, it responds to the same hormonal signals as your uterine lining. Each cycle, it thickens, breaks down, and bleeds. But unlike your uterine lining, the blood and debris have no way to exit. They accumulate inside a growing pocket on the ovary, producing inflammation that irritates surrounding tissue and promotes further cyst expansion. This means the cyst gets incrementally larger with every menstrual cycle, which is why endometriomas tend to grow steadily over months and years rather than appearing suddenly.
Dermoid Cysts and Slow Expansion
Dermoid cysts (mature cystic teratomas) are among the strangest structures in the body. They can contain hair, teeth, skin, and even fragments of neural tissue. They originate from germ cells, the precursors of eggs, that begin dividing on their own without fertilization. These cells retain the ability to differentiate into tissues from all three embryonic layers, which is why dermoid cysts produce such a bizarre mix of contents.
Genetic analysis shows that most dermoid cysts arise from a single germ cell after it completes the first stage of cell division (meiosis I), supporting what’s called a parthenogenetic origin. The resulting cells are diploid with homozygous genetic patterns, meaning they carry two copies of the same set of genes rather than the mixed set you’d see in a fertilized embryo.
These cysts grow slowly. A prospective study tracking dermoid cysts over roughly three years found that premenopausal women’s cysts grew at an average rate of 1.8 millimeters per year. In postmenopausal women, dermoid cysts showed no significant growth at all. This slow pace means a small dermoid can be monitored for years without intervention, particularly in women who want to preserve fertility. In the study, 28 women carried pregnancies to term without complications from their dermoid cysts.
Blood Supply and Fluid Accumulation
For any cyst to keep growing, it needs a blood supply and a way to accumulate fluid. Vascular endothelial growth factor (VEGF), a protein that stimulates the formation of new blood vessels, plays a role in both. VEGF is naturally active in the ovary during each cycle, supporting the rapid tissue remodeling that ovulation requires. In cysts, VEGF in the epithelial lining promotes both new vessel growth and increased vascular permeability, meaning blood vessels become leakier. This leakiness allows fluid to seep into the cyst cavity, contributing to its expansion.
When Thyroid Problems Trigger Cyst Growth
Severe hypothyroidism can cause ovarian cysts through a surprising cross-reaction. When your thyroid is severely underactive, your body produces very high levels of thyroid-stimulating hormone (TSH) in an attempt to push the thyroid to work harder. TSH is structurally similar enough to FSH that at high concentrations, it can weakly activate FSH receptors on the ovaries. This inappropriate stimulation can cause follicles to enlarge into cysts, sometimes dramatically. Some researchers have also found that certain women carry a mutation in their FSH receptor that makes it even more sensitive to TSH, amplifying the effect. Treating the underlying thyroid condition typically resolves the ovarian enlargement.
Size Thresholds and What They Mean
Simple cysts up to 3 centimeters are considered normal physiologic findings in women of reproductive age and don’t require monitoring. Cysts between 3 and 5 centimeters are almost certainly benign and also don’t need follow-up. Between 5 and 7 centimeters, yearly ultrasound monitoring is recommended. Above 7 centimeters, further imaging with MRI or surgical evaluation is typically considered, since ultrasound alone may not fully characterize the cyst.
For postmenopausal women, the thresholds are lower. Cysts up to 1 centimeter are clinically insignificant, found incidentally in up to 21% of women well past menopause. Between 1 and 7 centimeters, yearly ultrasound follow-up is recommended. Above 7 centimeters, additional imaging or surgery is considered.
Size alone doesn’t determine whether a cyst is dangerous, but it does shift the odds. Once a cyst exceeds 10 centimeters, there is roughly a 13% chance it is malignant. Imaging features matter as much as size: solid components, thick internal walls, and papillary projections (small finger-like growths inside the cyst) are the characteristics most associated with borderline or invasive tumors. A purely fluid-filled, smooth-walled cyst, even a moderately large one, is far less concerning than a smaller cyst with solid areas or irregular internal structures.