How Fast Can an Ovarian Cyst Actually Grow?

Ovarian cysts grow at wildly different rates depending on what they are, ranging from less than two millimeters per year for a slow-growing dermoid to a volume doubling every couple of months for a high-grade cancer. Most cysts that form during a normal menstrual cycle grow at roughly 1.4 mm per day, peak at ovulation, and then either collapse or linger for a few weeks before resolving on their own. The real question behind the title is usually a worried one, and the answer depends almost entirely on the type of cyst involved.

How a Normal Follicle Sets the Baseline

Every menstrual cycle, a follicle develops on one of the ovaries, swells with fluid, and releases an egg. That follicle grows at an average rate of about 1.4 mm per day during a natural cycle. Under fertility medications that stimulate the ovaries, the rate climbs to roughly 1.7 mm per day.¹ That means a follicle can go from barely detectable to about 2 cm across in under two weeks, which is completely normal. The follicle either ruptures at ovulation or, if something goes slightly off-script, sticks around and becomes what doctors call a functional cyst. This normal follicular growth rate is the yardstick against which all other ovarian cyst growth is measured.

Functional Cysts Usually Disappear on Their Own

Functional cysts are the most common type. They form when a follicle doesn’t rupture (a follicular cyst) or when the structure left behind after ovulation fills with fluid or blood (a corpus luteum cyst). Most of these resolve within the first few days of the next menstrual period. In one study tracking premenopausal women, the majority of cysts vanished at menstruation, and about two-thirds of those that lingered past the period were gone within three months, regardless of whether the women were on oral contraceptives.²

A larger study of over 2,600 women with newly discovered cysts found that about 63% resolved within roughly a year. Smaller cysts, under 3 cm, resolved faster than those larger than 6 cm. Simple, single-chambered cysts cleared faster than ones with internal dividing walls, though their overall resolution rates were similar.³ In the context of fertility treatment, where ovarian stimulation can produce multiple cysts, a clinical trial showed that roughly three-quarters resolved within one menstrual cycle whether women took oral contraceptives or simply waited. The ones that persisted after one cycle disappeared after a second without any treatment at all.⁴

So for the most common category of ovarian cyst, the growth story is really a shrinking story. The cyst appears, sometimes grows a bit over a cycle or two, and then the body reabsorbs it. The clinical approach is almost always watchful waiting: repeat the ultrasound in a few weeks and see if it’s gone.

Dermoid Cysts and Cystadenomas Grow Slowly

Dermoid cysts (mature cystic teratomas) are among the most distinctive ovarian growths. They contain tissue like hair, skin, and even teeth because they arise from germ cells that can differentiate into many tissue types. Unlike functional cysts, dermoids don’t resolve on their own. They just sit there and grow, slowly. In premenopausal women, the measured growth rate averages about 1.8 mm per year. In postmenopausal women, they essentially stop growing entirely, with some even shrinking slightly over time.⁵ At that pace, a dermoid discovered at 3 or 4 cm might take years to become large enough to cause symptoms or warrant surgery.

Serous cystadenomas, another common benign tumor, grow at a similar crawl. Research measuring their growth rate found an average of about 0.65 cm per year. That figure turns out to be roughly the same magnitude as the inherent measurement error on a standard ultrasound, which creates a practical problem: if you scan a cystadenoma six months after the first image, you often can’t tell whether it has actually grown or whether the slight size difference is just the ultrasound reading it differently. The researchers concluded that scans spaced more than a year apart are often needed to get meaningful information about whether a benign cystadenoma is truly growing.⁶ This is worth knowing if you’ve been told a cyst “grew a little” between two ultrasounds done only months apart. It may not have grown at all.

Endometriomas Tend to Hold Steady or Shrink

Endometriomas, sometimes called “chocolate cysts” because they’re filled with old blood, form when endometrial tissue grows on or inside the ovary. Their growth behavior is distinct from other cyst types. A study tracking 83 women with endometriomas over time found that nearly half experienced a reduction in cyst size, about a third stayed essentially the same, and only about one in five grew. The median change was actually a slight shrinkage of about 1.7 mm per year.⁷

That said, the range was wide. Some endometriomas shrank substantially, while a few grew by several centimeters over the study period. The unpredictable individual variation means watchful waiting works for many endometriomas, but ongoing monitoring remains important, especially in women who aren’t trying to conceive and for whom the cyst’s behavior over time influences treatment decisions.

When Cysts Grow Alarmingly Fast

At the opposite extreme from the slow-growing dermoid, some ovarian masses can expand at a pace that is visible to the naked eye over weeks. Mucinous cystadenomas, benign tumors that fill with thick mucus-like fluid, are capable of reaching enormous sizes. In one reported case, a 13-year-old girl developed a mucinous cystadenoma that caused noticeably rapid abdominal enlargement over about 20 days, with a dramatic size increase in the final three days before she reached the emergency department. The mass measured roughly 33 by 21 cm on a CT scan, filling most of her abdomen and pelvis.⁸ That growth rate dwarfs anything seen in dermoids or serous cystadenomas and illustrates why abdominal swelling that progresses over days to weeks needs urgent evaluation.

Giant ovarian cysts can also accumulate more quietly. Case reports describe women who attributed months of gradual abdominal growth and missed periods to pregnancy, only for an ultrasound to reveal a massive cyst.⁹ These cases usually involve benign tumors, but the sheer mass of fluid they accumulate can cause breathing difficulty, leg swelling, and abdominal pain long before the diagnosis is made. When a cyst has grown large enough that open surgery is required rather than laparoscopy, the delay in presentation is the main driver of complications.

How Fast Ovarian Cancer Actually Grows

The growth rate of malignant ovarian tumors is one of the reasons screening has proven so difficult. High-grade serous ovarian cancer, the most common and aggressive subtype, doubles in volume roughly every 2.2 months at the primary ovarian site. Metastatic deposits on the omentum (the fatty tissue that hangs in front of the intestines) double even faster, about every 1.8 months. In cases where researchers could estimate the timeline, the median interval between disease beginning in the ovary and metastasis spreading to the omentum was just over 13 months.¹⁰

To put that doubling time in perspective: a lesion that starts at 1 cm could reach roughly 4 cm in about six months through repeated volume doublings. That’s fast enough to go from essentially undetectable on routine imaging to advanced disease within a year. This is the core challenge with ovarian cancer detection. Annual screening with ultrasound or blood tests would need to catch a tumor during a narrow window, and many tumors would slip through the gap between screens. The rapid growth also explains why ovarian cancer is so often diagnosed at an advanced stage.

Rapid Growth as a Clinical Red Flag

Doctors use growth rate as one of the signals that an ovarian mass might not be benign. A retrospective study of 48 consecutive cases found that among masses that appeared benign on ultrasound and scored low on standard risk indices, the ones that grew rapidly, defined as increasing in size by more than 50% or first appearing within 6 to 12 months before surgery, were all found to be either borderline tumors or early-stage ovarian or tubal cancers. Among slowly growing masses in the same cohort, every one turned out to be benign.¹¹ This matters because ultrasound appearance alone missed these cancers; the growth behavior was the distinguishing clue.

This is one reason follow-up imaging exists. A cyst that looks simple and harmless on a single ultrasound might be fine, but a cyst that looks simple and doubles in size over six months is a different clinical story. French clinical guidelines, for instance, allow watchful waiting for a simple, fluid-filled cyst up to 10 cm in a non-pregnant woman with no cancer history, but a cyst that changes rapidly would prompt earlier surgical evaluation.¹²

Medications and Treatments That Trigger Cyst Formation

Some cysts don’t develop on their own but are nudged into existence by medications. The most dramatic example is ovarian hyperstimulation syndrome, a complication of fertility drugs. The medications used to stimulate egg development can push the ovaries into overdrive, causing them to swell dramatically and leak fluid into the abdomen. In severe cases, this leads to massive ovarian enlargement along with fluid in the abdomen and chest, blood clotting problems, and potentially life-threatening dehydration from fluid shifts.¹³ The ovaries can go from normal size to many times their usual volume within the span of a single treatment cycle.

Tamoxifen, a drug widely used in breast cancer treatment, also promotes cyst formation. Because tamoxifen blocks estrogen at the breast but can stimulate ovarian activity through a different pathway, it effectively tricks the body into ramping up ovarian function. In one study, nearly half of premenopausal women taking tamoxifen developed ovarian cysts, compared to very few postmenopausal women on the same drug. Cyst formation was linked to higher estrogen levels and to still having active menstrual cycles.¹⁴ Other research in Korean women found lower but still notable rates, and neither study found that standard chemotherapy independently raised the risk.¹⁵ The practical takeaway for women on tamoxifen is that ovarian cysts discovered during treatment are common and usually benign, but they still require monitoring to rule out other causes.

Cysts in Children and Adolescents

Ovarian cysts appear across the entire age range, including in fetuses, newborns, and children. Fetal ovarian cysts are typically detected on prenatal ultrasound and are managed conservatively unless they’re large. The risk of the cyst twisting on its blood supply (ovarian torsion) depends heavily on size: among fetal cysts in the 30 to 59 mm range, the torsion rate runs between roughly 15% and 34%.¹⁶ Larger cysts carry more risk simply because they have more leverage to twist.

In a multicenter study of 100 children and adolescents with ovarian cysts, most were simple cysts, but nine had ovarian tumors, and seven experienced torsion. Torsion occurred with both large and small cysts, though it was more common with larger ones.¹⁷ The growth rates of ovarian cysts in younger patients aren’t systematically different from those in adults for the same cyst type, but the clinical urgency can differ. In a child or teenager, the goal is to preserve as much ovarian tissue as possible for future fertility, which means doctors try hard to avoid removing an entire ovary. Rapid growth or signs of torsion change that calculation quickly.

Pregnancy and Hormonally Driven Ovarian Changes

Pregnancy creates its own hormonal environment that can affect ovarian cyst behavior. Corpus luteum cysts are common in early pregnancy and typically resolve by the second trimester as the placenta takes over hormone production. A rarer condition called hyperreactio luteinalis can cause the ovaries to develop multiple large cysts in response to the high levels of human chorionic gonadotropin (hCG) produced during pregnancy. These cysts can grow substantially, sometimes reaching sizes that cause abdominal discomfort or complicate delivery.¹⁸ The condition is most often seen with twin pregnancies, molar pregnancies, or any situation where hCG levels are unusually high. Despite the alarming appearance on ultrasound, hyperreactio luteinalis usually resolves after delivery.

French clinical guidelines recommend watchful waiting for simple cysts smaller than 6 cm discovered during pregnancy, reserving surgery for cysts that are complex, symptomatic, or growing rapidly.¹² Because the risk of surgery during pregnancy includes preterm labor, the threshold for intervention is higher than in non-pregnant women.

Why Ultrasound Measurements Can Be Misleading

One underappreciated wrinkle in the “how fast is it growing” conversation is measurement variability. Ultrasound is the workhorse tool for tracking ovarian cysts, but it’s operator-dependent and inherently imprecise. The same cyst scanned by two different technicians on the same day might yield measurements that differ by several millimeters. As mentioned earlier, cystadenomas grow at an average of about 0.65 cm per year, and the measurement error on ultrasound is a similar magnitude.⁶

This means that for slow-growing benign cysts, apparent “growth” between two scans done a few months apart may be nothing more than noise. If your doctor tells you a cyst went from 3.2 cm to 3.8 cm between two ultrasounds six months apart, that difference is within the range that could be entirely attributable to measurement variation. It does not necessarily mean the cyst grew. On the other hand, a cyst that goes from 3 cm to 6 cm in the same interval has clearly grown in a way no measurement error could explain, and that kind of change warrants closer attention. Understanding this distinction can save you from unnecessary anxiety, and help you ask the right questions at follow-up appointments.

The Fluid Accumulation Question

Most ovarian cysts are, at their core, fluid-filled sacs, and the rate at which they expand depends partly on how quickly fluid accumulates inside them. In functional cysts, the fluid is follicular fluid or blood, and production is governed by hormonal signals that typically self-limit. In cystadenomas, the lining cells secrete serous or mucinous fluid continuously, which is why these tumors can reach remarkable sizes if left alone for years. Research on the water-channel proteins (aquaporins) in ovarian tissue has found that the expression of these channels differs between normal and cancerous ovaries, suggesting that tumors may alter how efficiently fluid moves into the cyst cavity.¹⁹ This area of research is still early, and no one is using aquaporin levels to predict cyst growth in the clinic. But it points to a plausible biological explanation for why some cysts seem to inflate far more aggressively than others, even among the same tumor type.

Mucinous tumors, in particular, produce a thick, gel-like fluid that accumulates in large volumes. The lining cells essentially function as a mucus-producing factory, and because mucinous fluid is viscous and doesn’t reabsorb easily, it just keeps building up. This is why mucinous cystadenomas account for a disproportionate share of the case reports describing giant ovarian cysts weighing tens of kilograms. The cyst isn’t necessarily dividing cells faster than other tumor types; it’s filling with fluid faster than the body can clear it.