A slight difference in ovarian size is common and usually represents normal biological function. The ovaries are a pair of almond-shaped organs responsible for producing eggs and reproductive hormones. While they are typically similar in size, they are not perfect twins. Reasons for size differences range from predictable monthly hormonal shifts to specific medical conditions that require evaluation. This article explains the natural processes that cause temporary asymmetry and the pathological reasons that lead to a persistent or significant size disparity.
Natural Changes in Ovarian Size During the Menstrual Cycle
The most frequent reason for a temporary size difference between the two ovaries is the monthly ovarian cycle. Hormonal signals prompt a group of fluid-filled sacs, known as follicles, to begin developing. This development is not symmetrical; typically, only one ovary produces a dominant follicle destined to release an egg.
As this dominant follicle matures, it swells considerably, reaching a diameter of about 22 to 24 millimeters just before ovulation. This single, large, fluid-filled structure causes the producing ovary to temporarily increase in volume compared to its counterpart. Standard adult ovaries generally measure between 3 to 5 centimeters in length.
After the egg is released during ovulation, the ruptured follicle transforms into the corpus luteum. This temporary structure produces progesterone to support a potential pregnancy and can measure between 2 to 5 centimeters in diameter, maintaining the enlarged size of that ovary for several days. The corpus luteum eventually shrinks if pregnancy does not occur, and the ovarian size returns to its baseline. It is common for one ovary to be twice the volume of the other during different phases of the menstrual cycle.
Specific Medical Conditions Causing Ovarian Asymmetry
When an ovarian size difference is persistent or dramatic, it often points to a specific underlying condition. The most common cause of unilateral enlargement is the formation of ovarian cysts, which are fluid-filled sacs that grow larger than normal follicular or corpus luteum structures. Functional cysts, such as follicular cysts that fail to rupture or corpus luteum cysts that accumulate excess fluid, can cause temporary asymmetry lasting several weeks.
Polycystic Ovary Syndrome (PCOS) often leads to a generalized enlargement of both ovaries, but one ovary may appear more affected, resulting in asymmetry. This enlargement is caused by the accumulation of multiple small follicles, each typically measuring up to 8 millimeters, which are unable to mature and be released due to hormonal imbalances. The number of these small structures contributes to the overall increase in ovarian volume.
Endometriosis, a condition where tissue similar to the uterine lining grows outside the uterus, can cause significant asymmetry if it forms endometriomas, commonly known as “chocolate cysts.” These cysts are filled with old, dark blood and cause the affected ovary to swell noticeably.
Finally, non-cancerous (benign) or cancerous (malignant) tumors represent a less common but more serious cause of unilateral enlargement. These growths cause a persistent, solid mass that leads to a significant disparity in size between the ovaries. While tumor size does not always correlate with malignancy, growths larger than 8 centimeters often raise greater suspicion and require close monitoring.
When Size Differences Signal a Need for Medical Evaluation
While minor ovarian asymmetry is routine, certain accompanying symptoms suggest that the size difference is pathological and warrants immediate medical attention. Sudden, severe, and sharp pelvic pain, especially when accompanied by nausea and vomiting, is a symptom of ovarian torsion, a medical emergency where an enlarged ovary twists on its supporting ligaments, cutting off its blood supply.
Other concerning symptoms include persistent pelvic or abdominal pain that lasts for several weeks, unexplained bloating, or a feeling of being full quickly after eating. Changes in urinary habits, such as an increased urgency or frequency of urination, can also signal a mass pressing on the bladder. Any unexplained changes in the menstrual cycle, such as irregular bleeding or a sudden cessation of periods, should prompt a consultation.
The diagnostic process begins with a physical examination, followed immediately by a transvaginal or transabdominal ultrasound. The ultrasound is the primary tool for evaluating asymmetry, as it accurately measures the ovarian volume and assesses the internal structure of the enlarged ovary. The volume is calculated using a standard formula involving the three axial measurements (length, width, and depth).
To further evaluate the nature of the enlargement, a physician may order blood tests. These often include the CA-125 tumor marker, a protein that can be elevated in ovarian cancer, though it is also raised in benign conditions like endometriosis and fibroids. A normal CA-125 level is generally below 35 units per milliliter, but an elevated result only indicates the need for further imaging and investigation, not a definitive cancer diagnosis. Understanding the underlying cause of ovarian asymmetry is accomplished by combining the patient’s symptoms, the physical exam, the detailed ultrasound findings, and the results of any relevant blood work.