An enlarged ovary almost always traces back to something growing inside or on it, whether that is a fluid-filled cyst, a benign mass, inflamed tissue, or, less often, a malignant tumor. The most common explanation by far is a functional cyst tied to your normal menstrual cycle, which typically resolves on its own within a few weeks. But the list of possibilities is long enough that the finding deserves a closer look, because “enlarged ovary” is a description, not a diagnosis, and the cause shapes everything from whether you need treatment to how urgently you need it.
Functional Cysts From Your Menstrual Cycle
Every month, one of your ovaries grows a small fluid-filled sac called a follicle to house the developing egg. That follicle can reach roughly two centimeters before it ruptures at ovulation. Sometimes the follicle does not rupture and keeps growing, creating what is called a follicular cyst. Other times ovulation happens normally but the leftover structure, the corpus luteum, fills with fluid or blood and forms a corpus luteum cyst. Both of these are classified as functional cysts because they arise from normal ovarian activity.
Ultrasound studies tracking the ovary across the cycle have documented how a cystic structure resembling a corpus luteum can appear in the second half of the cycle, and when the follicle never clearly collapses beforehand, it may represent a luteinized unruptured follicle rather than a true ovulation event.1Ultrasound in Medicine & Biology. Ovulation and corpus luteum formation observed by ultrasonography These cysts can make one ovary appear noticeably bigger on an ultrasound and sometimes cause a dull ache on one side. Most resolve within one to three menstrual cycles without any treatment. They are so common that finding one is almost routine in reproductive-age women getting a pelvic ultrasound.
What Counts as “Normal” Ovarian Size
To know whether an ovary is enlarged, you need a baseline, and that baseline shifts with age. A large normative study found that average ovarian volume rises from about 0.7 mL at age two to a peak of roughly 7.7 mL around age twenty, then gradually declines to about 2.8 mL by menopause, with even smaller volumes afterward.2PubMed Central. Ovarian volume throughout life: a validated normative model So a 20-year-old with an ovary measured at 8 mL is within the expected range, while the same measurement in a 55-year-old postmenopausal woman would warrant further investigation. A functional cyst in a younger person can temporarily push that volume well above normal and still be completely benign.
Polycystic Ovary Syndrome
Polycystic ovary syndrome, or PCOS, is one of the most well-known causes of persistently enlarged ovaries. Rather than one dominant follicle maturing each cycle, multiple small follicles stall at an early stage and line the outer rim of the ovary, giving it a characteristic “string of pearls” look on ultrasound. The ovaries often grow beyond normal volume because of these accumulated follicles combined with changes in the surrounding tissue.
The underlying problem involves the ovary’s hormone-producing cells. Research into the theca cells of women with classic PCOS has shown that these cells overexpress several steroidogenic enzymes, particularly one called cytochrome P450c17, leading to excess androgen production.3PubMed Central. The Pathogenesis of Polycystic Ovary Syndrome (PCOS): The Hypothesis of PCOS as Functional Ovarian Hyperandrogenism Revisited That hormonal imbalance is what drives the irregular periods, acne, and excess hair growth many people with PCOS experience. Because the ovarian enlargement in PCOS is ongoing rather than temporary, it tends to show up consistently on imaging, unlike a functional cyst that comes and goes.
Endometriomas
Endometriosis can directly affect the ovary by forming a type of cyst called an endometrioma, sometimes nicknamed a “chocolate cyst” because of the dark, old blood it contains. These form when endometrial-like tissue invades the ovary or when surface endometriosis bleeds into the ovarian cortex.4PubMed Central. Pathophysiology and Clinical Implications of Ovarian Endometriomas Endometriomas can range from small to several centimeters across and often cause chronic pelvic pain, painful periods, and fertility problems. On ultrasound they have a distinct “ground glass” appearance that helps distinguish them from simple fluid-filled cysts, though sometimes further imaging or surgery is needed for a definitive diagnosis.
Benign Ovarian Tumors
Not every solid or complex mass in an ovary is cancer. Several types of benign tumors can enlarge the ovary, and dermoid cysts (also called mature cystic teratomas) are among the most common. These unusual growths contain tissue types like hair, skin, teeth, or fat, because they arise from germ cells capable of developing into different tissues. A prospective study tracking dermoid cysts over time confirmed them as benign in all surgically removed cases, with histologic examination verifying mature cystic teratomas.5PubMed. The growth pattern of ovarian dermoid cysts: a prospective study in premenopausal and postmenopausal women Other benign tumors include cystadenomas, which are fluid-filled growths arising from the ovary’s surface, and fibromas, which are solid. Many benign tumors grow slowly and can reach a surprising size before they cause symptoms.
Infections and Tubo-Ovarian Abscess
Pelvic inflammatory disease, usually caused by sexually transmitted bacteria traveling upward from the cervix, can lead to one of the more serious forms of ovarian enlargement: a tubo-ovarian abscess. This is a pocket of infected fluid that involves the fallopian tube and ovary, often fusing them together. Tubo-ovarian abscesses represent a severe form of PID and carry high morbidity.6The Obstetrician & Gynaecologist. Diagnosis and management of tubo‐ovarian abscesses When diagnosis or treatment is delayed, complications like sepsis and peritonitis can follow, significantly affecting reproductive health.7Germs. Tubo-Ovarian Abscess, Sepsis and Diffuse Peritonitis in Pelvic Inflammatory Disease—A Diagnostic and Therapeutic Review
Symptoms typically include fever, lower abdominal pain, and sometimes abnormal vaginal discharge. The ovary on the affected side will appear enlarged and inflamed on imaging. Treatment usually starts with intravenous antibiotics, though some abscesses require drainage or surgical intervention if they do not respond.
Ovarian Torsion
Torsion happens when the ovary twists on its blood supply, cutting off circulation. It is considered a gynecologic surgical emergency. The twist often occurs because a cyst or tumor has made the ovary heavy enough to rotate, though torsion can also happen after ovarian hyperstimulation.8Springer. Does gradual detorsion protect the ovary against ischemia-reperfusion injury in rats? The hallmark is sudden, severe one-sided pelvic pain, often with nausea and vomiting. On imaging the ovary appears enlarged and swollen because blood can still trickle in through arteries but cannot drain properly through the compressed veins. Prompt surgical untwisting (detorsion) is important to save the ovary and preserve fertility. Ironically, the tissue damage from restoring blood flow can be more severe than the damage from the blockage itself, because of oxygen-derived free radicals generated during reperfusion.
Pregnancy-Related Enlargement
Pregnancy itself can enlarge the ovaries in ways that sometimes alarm both patients and clinicians. A corpus luteum cyst of pregnancy is normal in the first trimester and supports hormone production until the placenta takes over. More dramatic is a condition called hyperreactio luteinalis, in which both ovaries develop multiple large cysts, often discovered in the third trimester.9PubMed Central. Hyperreactio luteinalis: An often mistaken diagnosis The ovaries can swell to an impressive size, and the condition sometimes triggers maternal androgen excess.10PubMed Central. Sonography Incidentally found hyperreactio luteinalis in pregnancy
The trouble is that hyperreactio luteinalis can look worrying on imaging. Unilateral cases in particular can mimic ovarian malignancy, leading to potential misdiagnosis and unnecessary surgery if the clinical team is not aware of the condition.11Medicine. Unilateral hyperreactio luteinalis of the ovary during pregnancy mimics ovarian malignancy: A case report Despite the alarming appearance, hyperreactio luteinalis is benign and typically resolves on its own after delivery.
Fertility Treatments and Ovarian Hyperstimulation Syndrome
If you are undergoing fertility treatment, ovarian enlargement is sometimes an expected side effect and sometimes a complication. Medications used to stimulate egg production intentionally push the ovaries to develop multiple follicles at once. In most cycles this leads to mildly enlarged ovaries that return to normal afterward. But in ovarian hyperstimulation syndrome (OHSS), the response is excessive: the ovaries become markedly cystic and enlarged, and fluid shifts out of blood vessels into the abdomen and other body cavities because of increased capillary permeability.12PubMed Central. Ovarian hyperstimulation syndrome Mild OHSS causes bloating and discomfort; severe cases can lead to blood clots, kidney problems, and require hospitalization. The ovaries in severe OHSS can reach many times their normal size.
Drug-Induced Ovarian Cysts
Fertility drugs are not the only medications that can enlarge the ovaries. Tamoxifen, widely used in breast cancer treatment, has been linked to ovarian cyst formation in premenopausal patients. The mechanism appears to involve tamoxifen’s complex hormonal effects: despite being classified as an anti-estrogen, it can paradoxically stimulate the ovaries in women whose ovaries still respond to FSH.13PubMed Central. Ovarian cysts in women receiving tamoxifen for breast cancer Studies have found that premenopausal breast cancer patients on tamoxifen may develop ovarian overstimulation, cystic formations, and fibroid overgrowth.14PubMed Central. Tamoxifen treatment in premenopausal breast cancer patients may be associated with ovarian overstimulation, cystic formations and fibroid overgrowth If you are taking tamoxifen and develop pelvic symptoms, your oncologist will likely want to check whether ovarian cysts have formed.
Thyroid Problems and Other Endocrine Causes
The connection between the thyroid and the ovaries is not intuitive, but severe, longstanding hypothyroidism can enlarge the ovaries by an unusual hormonal crosstalk. In a condition known as Van Wyk-Grumbach syndrome, very high levels of thyroid-stimulating hormone (TSH), which build up when the thyroid is underperforming, can activate FSH receptors on the ovary because the two hormones share a structural component.15Journal of the Endocrine Society. Precocious Puberty in Hypothyroidism: Mini-Review of Van Wyk–Grumbach Syndrome The result is ovarian hyperstimulation that produces multicystic, enlarged ovaries and elevated estrogen, sometimes triggering precocious puberty in children or menstrual irregularities in adults.16PubMed Central. Van Wyk-Grumbach syndrome: The importance of thyroid function tests in a child presenting with multicystic ovaries The good news is that treating the hypothyroidism with thyroid hormone replacement typically causes the ovarian cysts to shrink and resolve without surgery.
Genetic Conditions That Affect the Ovaries
Rarely, ovarian enlargement in a child turns out to be related to a genetic condition. McCune-Albright syndrome involves a mutation that causes certain tissues to become overactive without the usual hormonal signals. In the ovaries, this can lead to large solitary cysts that produce estrogen independently, causing signs of puberty in very young girls. A study measuring ovarian volumes in girls with McCune-Albright syndrome found that mean ovarian volume was significantly greater than in normal prepubertal girls, with the asymmetry largely driven by a big solitary cyst in one ovary.17PubMed. Ovarian function in girls with McCune-Albright syndrome An ovarian cyst accompanied by precocious puberty in a child should prompt evaluation for this syndrome as well as for granulosa cell tumors.18PubMed Central. Precocious pseudopuberty due to an autonomous ovarian follicular cyst: case report with a review of literatures
Ovarian Cancer
This is the concern that often drives the question in the first place, so it is worth addressing honestly: ovarian cancer can cause an enlarged ovary, but it is far less common than the benign causes listed above. The challenge is that early-stage ovarian cancer often produces no symptoms at all, or only vague ones like bloating and feeling full quickly. When imaging reveals a complex mass with both solid and cystic components, irregular borders, or internal blood flow, the suspicion for malignancy rises. MRI is particularly good at distinguishing benign from malignant ovarian masses because it can identify different tissue types, including hemorrhage, fat, and collagen within a mass.19Journal of Oncology. Benign and Suspicious Ovarian Masses—MR Imaging Criteria for Characterization: Pictorial Review A postmenopausal woman with a new solid ovarian mass gets a faster workup than a 25-year-old with a simple cyst, because the baseline risk changes dramatically with age and menopausal status.
How an Enlarged Ovary Gets Investigated
Transvaginal ultrasound is the standard first step when an enlarged ovary is found or suspected. Multiple national guidelines agree on its central role in the initial evaluation of an adnexal mass.20PubMed. Management of adnexal mass: A comparison of five national guidelines Ultrasound can characterize a mass as simple (fluid-filled with thin walls, usually benign), complex (mixed solid and cystic, needs more evaluation), or solid. Color Doppler imaging, which maps blood flow, has been studied as an additional tool in conditions like PCOS, where increased blood flow within the ovary can help support the diagnosis.21American Journal of Obstetrics and Gynecology. The role of color doppler imaging in the diagnosis of polycystic ovary syndrome
When ultrasound leaves the picture unclear, MRI is widely accepted as the next step for indeterminate masses. Blood tests such as CA-125 are sometimes drawn, though guidelines note that CA-125 is not reliable as a screening tool for ovarian cancer and its role even in evaluating a known mass remains debated.20PubMed. Management of adnexal mass: A comparison of five national guidelines Risk prediction models that combine imaging features with blood markers help clinicians decide who needs surgery and who can safely be monitored.
For small, simple cysts, watchful waiting is often appropriate. French guidelines, for example, consider observation a valid approach for a unilocular, symptom-free cyst under 10 cm in a woman with no cancer history.22PubMed. Management of presumed benign ovarian tumors: updated French guidelines When surgery is needed, laparoscopy is generally preferred for masses believed to be benign.
When It Is Not Actually the Ovary
Sometimes what appears to be an enlarged ovary on initial assessment turns out to be something else nearby. Peritoneal inclusion cysts, for instance, are uncommon fluid collections that form around a normal ovary, typically in women who have had prior pelvic surgery or inflammation. On imaging they can look remarkably like an ovarian tumor, leading to frequent misdiagnosis.23PubMed Central. Multilocular peritoneal inclusion cyst mimicking an ovarian tumor: A case report Paratubal cysts (arising from tissue next to the fallopian tube) and hydrosalpinx (a fluid-filled, blocked tube) can also be mistaken for ovarian enlargement. Careful ultrasound technique and, when needed, MRI usually sort these out, but the possibility of a mimic is worth knowing about, especially if you have been told your ovary looks enlarged and a repeat scan shows a different picture.
Ovarian Remnant Syndrome After Surgery
If you have had one or both ovaries removed and are still having pelvic pain or hormonal symptoms, ovarian remnant syndrome is an underrecognized possibility. It happens when a small piece of ovarian tissue is inadvertently left behind during oophorectomy, often because dense adhesions from endometriosis, prior surgeries, or PID made full removal difficult. That remnant can respond to hormonal stimulation, grow, form cysts, and cause pain that mimics the original condition.24PubMed Central. Ovarian remnant syndrome: an unsuspected diagnosis The remnant may show up as a small pelvic mass on imaging, and measurable hormone levels (estrogen, FSH) in someone who should have none after bilateral oophorectomy are a strong clue. Treatment typically requires another surgery to remove the residual tissue, ideally by a surgeon experienced with the condition.