The ovaries are paired reproductive organs that reside in the pelvis on either side of the uterus. Their primary biological role is to produce oocytes, or eggs, for reproduction and to secrete the sex hormones estrogen and progesterone. When a medical scan detects that one of these organs is noticeably larger than the other, it can cause concern. However, some degree of size variation between the two is a common physiological finding. Interpreting this size difference requires context, as it can reflect anything from a normal temporary monthly change to an underlying condition.
Normal Ovary Size and Measurement
The size of a healthy ovary changes naturally throughout a woman’s life, particularly during her reproductive years. Typically, the adult ovary measures three to five centimeters in length, translating to an average volume of three to six cubic centimeters (cc) during the non-ovulatory phase. Measurements are most often taken using a transvaginal pelvic ultrasound.
A slight disparity in size between the right and left ovary is frequently observed and is usually of no clinical concern. It is not uncommon for one ovary to be up to twice the volume of the other without indicating any disease process. Significant enlargement, however, is generally defined as a volume greater than 20 cc, which warrants further investigation.
Common Functional Causes of Asymmetry
The most frequent reasons for temporary ovarian asymmetry are directly related to the normal process of the menstrual cycle. Throughout the cycle, one ovary is typically more active than the other, preparing an egg for release. This activity leads to the formation of functional cysts, which are fluid-filled sacs that are a harmless byproduct of ovulation.
Follicular Cysts
Follicular cysts develop when a dominant follicle matures but fails to rupture and release the egg, continuing to grow instead. These cysts usually form in the first half of the cycle, can reach 22 to 24 millimeters, and temporarily enlarge that particular ovary. They are generally thin-walled structures that resolve spontaneously within a few weeks or months.
Corpus Luteum Cysts
Corpus luteum cysts form after ovulation has successfully occurred. Following the release of the egg, the follicle remnants transform into the corpus luteum, which produces progesterone. If this structure fills with fluid or blood, it becomes a cyst, leading to a temporary size difference in the luteal phase of the cycle. Functional cysts are the most common explanation for a unilateral size difference found incidentally on a pelvic scan.
Pathological Conditions and Clinical Significance
When the size difference is significant, persistent, or accompanied by symptoms, it may be due to a pathological process. Ovarian torsion is a time-sensitive medical emergency that occurs when the ovary twists around its supporting ligaments, cutting off its blood supply. This twisting causes acute, severe lower abdominal pain, often accompanied by nausea and vomiting, and rapid unilateral enlargement due to swelling and fluid buildup.
Chronic Conditions
Certain chronic conditions can also lead to non-resolving unilateral enlargement. Endometriomas, often called “chocolate cysts,” form when tissue similar to the lining of the uterus grows on the ovary. This tissue bleeds during the menstrual cycle, filling the cyst with old blood and causing chronic enlargement in one or both ovaries. Polycystic Ovary Syndrome (PCOS) can also cause enlargement, though it usually affects both ovaries, as the presence of many small follicles increases the overall volume.
Neoplasms
Any persistent or rapidly growing unilateral mass raises concern for an ovarian neoplasm, which can be either benign or malignant. Benign tumors, like dermoid cysts (teratomas), can grow quite large and cause significant asymmetry. Malignant tumors are the most serious cause of unilateral enlargement and are often complex in appearance on imaging. They may present with non-specific symptoms like bloating and pelvic pressure.
Diagnostic Steps and Management
A detailed evaluation, typically beginning with a pelvic ultrasound, is necessary after detecting an ovarian size difference. The ultrasound helps characterize the enlarged ovary by determining if the structure is a simple, fluid-filled cyst or a complex mass with solid components. Features like thick septations, internal nodules, or blood flow patterns are assessed to estimate the likelihood of a benign versus a pathological cause.
For simple cysts less than five centimeters in a premenopausal woman, the typical management is watchful waiting, involving a repeat ultrasound in six to twelve weeks. Blood tests, such as the CA-125 tumor marker, may be used, particularly in postmenopausal women or when complex features are present. This marker can also be elevated by benign conditions like endometriosis.
Management depends entirely on the confirmed diagnosis. Functional cysts are observed, while conditions like PCOS may be managed with hormonal therapy. Torsion requires immediate surgical intervention to untwist the ovary and restore blood flow. Large, persistent, or suspicious masses may necessitate surgical removal for definitive diagnosis and treatment.