Having two ovaries that measure differently on an ultrasound is, in most cases, completely normal. The ovaries are dynamic organs that change size throughout every menstrual cycle, across life stages, and in response to medications, and at any given moment one will almost always be a bit larger than the other. A large ultrasound study found no significant differences in volume between right and left ovaries on average, but that average masks the reality that one ovary is typically in the middle of growing a follicle or housing a corpus luteum while the other is not. Still, there are situations where a notable size gap signals something worth investigating.
Why Your Ovaries Are Rarely the Same Size at Any Given Moment
Each month, one ovary “wins” the race to ovulate. A dominant follicle grows to roughly two centimeters before releasing an egg, and that follicle alone can temporarily double or triple the apparent volume of the ovary hosting it. After ovulation, the collapsed follicle transforms into a corpus luteum, a temporary hormone-producing structure that grows in size during the first six or so days after ovulation before gradually shrinking.1PubMed Central. Form and function of the corpus luteum during the human menstrual cycle During that window, the ovary with the corpus luteum is measurably bigger than the other.
This alternation is somewhat random. Ovulation does not strictly alternate sides from month to month the way many people assume. One ovary can ovulate several cycles in a row. So if you happen to get an ultrasound during the second half of your cycle, you might see one ovary looking plump and the other looking comparatively small, and that would be unremarkable. A study of ovarian volumes confirmed no consistent right-versus-left difference across groups of menstruating, postmenopausal, and premenarchal women, even when researchers controlled for weight and menstrual-cycle phase.2PubMed. Ovarian volumes measured by US: bigger than we think The takeaway: on average, neither ovary is inherently larger, but at any single snapshot in time, a difference is expected.
How Ovarian Size Changes with Age
Ovarian volume follows a predictable arc over a lifetime. A validated model of ovarian volume across age groups found that the average ovary 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 and even smaller volumes afterward.3PubMed Central. Ovarian volume throughout life: a validated normative model Age alone accounted for about 69% of the variation in ovarian size in that analysis, which is a remarkably large share for a single variable.
A separate large study broke the numbers down by decade: average ovarian volume was about 6.6 cm³ for women under 30, dropping to around 4.8 cm³ in the forties and down to about 2.1 cm³ in the sixties.4PubMed. Ovarian volume related to age After menopause, the mean volume was roughly 2.2 cm³ compared to about 4.9 cm³ premenopausally. This decline accelerates around age 44 when adjusted for menopausal status.5PubMed. Determinants of ovarian volume in pre-, menopausal transition, and post-menopausal women: a population-based study
Why does age matter for size differences between your two ovaries? Because the smaller your ovaries get overall, the more noticeable a temporary follicular cyst or other growth becomes in relative terms. A one-centimeter cyst on a seven-milliliter premenopausal ovary barely registers; the same cyst on a two-milliliter postmenopausal ovary makes it look dramatically bigger than the other side. Clinicians factor in your age when deciding whether a size difference is worth a second look.
Functional Cysts and Corpus Luteum Cysts
The most common reason for a noticeable size difference is a functional cyst, meaning a cyst that arises from the normal process of ovulation. These come in two flavors. A follicular cyst forms when a dominant follicle fails to rupture and keeps growing, sometimes reaching three to five centimeters. A corpus luteum cyst forms when the corpus luteum fills with fluid or blood instead of reabsorbing on schedule. Both types typically resolve on their own within one to three menstrual cycles.
A hemorrhagic ovarian cyst, where bleeding occurs into either type of functional cyst, can cause sudden pain and make one ovary appear substantially enlarged on imaging. These cysts can look alarming on CT or MRI but are generally self-limiting.6SpringerOpen / Insights into Imaging. Cross-sectional imaging of acute gynaecologic disorders: CT and MRI findings with differential diagnosis-part I: corpus luteum and haemorrhagic ovarian cysts, genital causes of haemoperitoneum and adnexal torsion The key clinical question is whether the cyst has ruptured and is causing internal bleeding, which sometimes requires intervention, versus sitting there being painful but stable.
Endometriomas and Dermoid Cysts
Unlike functional cysts, some ovarian growths do not resolve on their own and can create a persistent size difference between the two ovaries.
Endometriomas, sometimes called “chocolate cysts,” form when endometrial-like tissue grows on or inside an ovary. They are filled with old blood and can range from small nodules to large masses. Beyond simply making one ovary bigger, the endometrioma appears to cause real damage to surrounding ovarian tissue. Research shows that follicular density in the tissue around an endometriotic cyst is significantly lower than in healthy ovaries, and this damage is not just from the mechanical stretching of a growing cyst. Substances like reactive oxygen species and proteolytic enzymes leaking from the cyst gradually replace healthy ovarian cortex with fibrous tissue.7Human Reproduction Update. The distinguishing cellular and molecular features of the endometriotic ovarian cyst: from pathophysiology to the potential endometrioma-mediated damage to the ovary This means an endometrioma is not just a cosmetic finding on an ultrasound; it can reduce the egg reserve in the affected ovary over time.
Dermoid cysts (mature cystic teratomas) are another common benign growth. These oddball cysts can contain hair, teeth, skin, and fat because they arise from germ cells capable of forming various tissue types. They grow slowly but can get impressively large. A case report documented a 13-by-12-centimeter dermoid cyst in an eight-year-old, with no evidence of malignancy.8PubMed Central. Pediatric puzzle: Large ovarian dermoid cyst and markedly elevated CA 19-9 in an 8-year-old Dermoid cysts generally do not damage surrounding ovarian tissue the way endometriomas do, but they can grow large enough to cause torsion (the ovary twisting on its blood supply). When surgery is needed, cyst size influences the approach; cysts larger than about 12 centimeters were more likely to require open surgery rather than laparoscopic removal in one study.9PubMed Central. Ovarian dermoid cysts: Does Size Matter When it Comes to Laparoscopic Surgery?
PCOS and Enlarged Ovaries
Polycystic ovary syndrome can make one or both ovaries appear enlarged and contain many small follicles. The “polycystic” label is a bit misleading, since the follicles are not true cysts but rather small antral follicles that have stalled in development. The ultrasound criteria for PCOS-type ovarian morphology are not one-size-fits-all. Research shows the diagnostic thresholds need to be adjusted for age: an ovarian volume of 12 mL and 13 or more follicles may indicate PCOS in a woman under 25, but the cutoffs drop to about 8 mL and 10 follicles for women in their late thirties, and down to 6 mL and 7 follicles for women over 44.10PubMed Central. Polycystic Ovary Morphology: Age-Based Ultrasound Criteria An older study proposed a somewhat higher volume cutoff of about 13 mL and also emphasized the ratio of ovarian stroma to total area as a useful marker.11PubMed. A new ultrasound criterion for the diagnosis of polycystic ovary syndrome: the ovarian stroma/total area ratio
In PCOS, the enlargement often affects both ovaries, but not always symmetrically. One ovary may have more follicles or a thicker stroma than the other. If your ultrasound shows one ovary meeting polycystic-morphology criteria and the other looking normal, your doctor will combine that finding with blood work and symptom history before making a diagnosis. Polycystic morphology on ultrasound alone does not equal PCOS; plenty of otherwise healthy women have ovaries that look polycystic on a scan.
How Hormonal Contraception Affects Ovarian Size
If you are on hormonal birth control, that alone can explain why your ovaries look different from what you expected or different from each other. Combined oral contraceptives suppress ovulation by keeping follicle-stimulating hormone and luteinizing hormone levels low, which prevents follicles from growing. This shrinks both ovaries substantially. A study comparing women on oral contraceptives, women using a copper IUD, and women using no contraception found that ovarian volumes were smallest in the oral-contraceptive group and largest in the IUD group. Women not on oral contraceptives showed volume changes across the menstrual cycle, with the larger ovary increasing in size up to about day 19 before declining. In contrast, women on oral contraceptives showed no cyclic variation at all, and both ovaries stayed at similarly reduced volumes throughout the cycle.12PubMed. Ovarian volume in gynecologically healthy women using no contraception, or using IUD or oral contraception
Research suggests that combined hormonal methods (the pill and the vaginal ring) can reduce ovarian volume by close to half compared to nonusers. Progestin-only methods like mini-pills and hormonal IUDs do not appear to have the same shrinking effect. If you recently stopped combined hormonal contraception, your ovaries may look temporarily smaller than expected, and one may “wake up” and resume follicular activity before the other, creating a transient size difference.
Fertility Treatments and Temporary Ovarian Enlargement
Controlled ovarian stimulation, the hormone injections used during IVF and some other fertility treatments, deliberately makes the ovaries grow multiple large follicles at once. This can massively enlarge one or both ovaries. In mild cases, the ovaries swell and feel tender. In severe cases, the result is ovarian hyperstimulation syndrome, or OHSS, where the ovaries become hugely enlarged and fluid leaks out of the bloodstream into the abdominal cavity.13PubMed Central. Ovarian hyperstimulation syndrome The enlargement can persist for months if pregnancy occurs, since the rising hCG from the pregnancy continues to stimulate the ovaries.14PubMed Central. Persistent megalocystic ovaries after ovarian hyperstimulation syndrome in a postpartum patient with polycystic ovarian syndrome: a case report
If both ovaries respond unevenly to stimulation medications, which is common, you can end up with a substantial size difference between them. Fertility specialists monitor this closely with serial ultrasounds and adjust dosing accordingly, but some asymmetry is expected.
What Ovarian Surgery Does to Size
If you have had a cyst removed from one ovary, the operated ovary often ends up smaller than the untouched one. A study tracking ovarian volume after laparoscopic cyst removal found that the operated ovary progressively shrank at follow-up, with a median volume reduction of about 33% compared to the contralateral intact ovary.15PubMed. Ovarian recovery after laparoscopic enucleation of ovarian cysts: insights from echographic short-term postsurgical follow-up This reduction reflects the loss of healthy tissue that gets removed along with the cyst wall during stripping procedures. The clinical implication is real: if you have had surgery on one ovary, a persistent size difference between the two is an expected consequence of the procedure, not a new problem.
Research into different surgical techniques for endometrioma removal has looked at ways to minimize this tissue loss, including three-stage approaches that combine drainage, medication, and then surgery. The goal is to preserve as much residual ovarian volume and follicle count as possible.16PubMed. Sonographic changes after laparoscopic cystectomy compared with three-stage management in patients with ovarian endometriomas: a prospective randomized study If you are facing ovarian surgery and fertility matters to you, the technique used and the surgeon’s experience can influence how much ovarian reserve is preserved.
When a Size Difference Points to Something Urgent
A few scenarios turn an asymmetric ovary finding into a medical concern worth acting on quickly.
Ovarian torsion occurs when an ovary twists on its supporting ligament and blood supply. It typically happens when an ovary is already enlarged by a mass, which gives it more weight to rotate. Torsion is found in roughly 2% to 15% of patients undergoing surgery for adnexal masses.17PubMed Central. A review of ovary torsion The hallmark symptom is sudden, severe, one-sided pelvic pain, often with nausea and vomiting. The twisted ovary swells rapidly because blood can get in through the arteries but cannot drain out through the veins. This is a surgical emergency; if blood flow is not restored, the ovary can die.
Pelvic inflammatory disease, usually from a sexually transmitted infection, can cause one or both ovaries to swell and become tender. On MRI, the findings include inflammation of the ovary (oophoritis) along with inflammation of the fallopian tubes and surrounding soft tissues.18PubMed. MRI in pelvic inflammatory disease: a pictorial review If the infection is one-sided or more severe on one side, you can end up with a noticeable size difference. PID requires antibiotic treatment and sometimes hospitalization, so an enlarged, painful ovary combined with fever, abnormal discharge, or pain during intercourse warrants prompt evaluation.
Ovarian tumors, both benign and malignant, create one-sided enlargement. In postmenopausal women, a virilizing ovarian tumor (one that produces excess androgens) can cause symptoms like new facial hair growth and voice deepening. These tumors tend to produce higher testosterone levels than ovarian hyperthecosis, a non-tumor condition that can look similar, though there is considerable overlap in hormone levels between the two. Pelvic MRI can reliably distinguish them.19European Journal of Endocrinology. Discriminating between virilizing ovary tumors and ovary hyperthecosis in postmenopausal women: clinical data, hormonal profiles and image studies
Symptoms That Should Prompt a Visit
Most ovarian size differences produce no symptoms and are found incidentally on imaging done for other reasons. But certain symptom patterns are worth knowing about because they overlap with ovarian cancer, even though each individual symptom is common and usually benign.
Research comparing women with ovarian cancer to women seen in primary care clinics found that the combination of bloating, increased abdominal size, and urinary symptoms occurred in 43% of women with cancer but only 8% of primary care patients. The strongest individual associations were increased abdominal size and bloating.20JAMA. Frequency of Symptoms of Ovarian Cancer in Women Presenting to Primary Care Clinics A separate study developing a symptom index found that pelvic or abdominal pain, increased abdominal size or bloating, and difficulty eating or feeling full early were independently associated with ovarian cancer when those symptoms were present for less than a year and occurred more than about 12 days per month.21PubMed. Development of an ovarian cancer symptom index: possibilities for earlier detection
The frequency and recency matter more than the symptoms themselves. Occasional bloating is universal. Bloating that is new, that happens most days of the month, and that has been escalating over weeks to months is the pattern that warrants investigation. The same applies to unexplained pelvic pain and urinary urgency. If you have those symptoms alongside an ultrasound finding of one enlarged ovary, that combination deserves follow-up imaging and possibly blood work.
When One Ovary Is Missing Entirely
Rarely, an ultrasound reveals only one ovary. Congenital absence of an ovary, where one ovary simply never developed, is uncommon but documented. A systematic review of ovarian absence found that the term “absence” is more accurate than “agenesis” in most cases, because the missing ovary likely did form during embryonic development but was lost to an event like torsion or vascular infarction before or shortly after birth, rather than failing to develop from the start. Most cases of unilateral ovarian absence do not come with other genital or urinary abnormalities, which supports this mechanical explanation over a developmental one.22PubMed Central. Ovarian absence: a systematic literature review and case series report
Women with one functioning ovary generally have normal fertility and hormone levels, since the remaining ovary compensates. But if the solo ovary develops a large cyst or other mass, there is no backup, which makes monitoring and conservative management even more important. Surgeons are especially cautious about preserving tissue when operating on the only ovary a patient has.
What Imaging Can and Cannot Tell You
Transvaginal ultrasound is the standard tool for measuring ovarian size, and it is quite good at distinguishing simple fluid-filled cysts from complex or solid masses. But the measurements are not perfectly precise. Different imaging modalities give slightly different numbers: one comparative study found that ovarian volume measured by 3D MRI was about half a milliliter smaller than the volume measured by 2D transvaginal ultrasound, while 2D MRI readings were substantially smaller than both.23Clinical Radiology. Using transvaginal ultrasonography and MRI to evaluate ovarian volume and follicle count of infertile women: a comparative study If you have had imaging done at two different facilities or with two different methods and the numbers look different, part of that gap may be the technology rather than a real change in your ovaries.
Ultrasound is also operator-dependent. The person holding the probe, the angle of approach, how full your bladder is, and whether the ovary is hiding behind a loop of bowel can all affect the measurement. A difference of a milliliter or two between the left and right ovary on a single scan is within the margin of normal measurement variability and almost never means anything. Clinicians get concerned when one ovary is several times larger than the other, when a mass has complex features like solid components or internal blood flow, or when an ovary that should have shrunk with menopause has grown instead.