The two oval-shaped ovaries are integral to the female reproductive system, performing the dual function of producing reproductive hormones and releasing eggs. These glands, situated on either side of the uterus, store all the egg cells a woman will ever have within tiny sacs called follicles. Ovulation is the process where a mature egg is released from one of the ovaries, typically occurring around the middle of the menstrual cycle, regulated by a complex interplay of hormones.
The Strict Alternation Myth
The idea that the ovaries take strict, alternating turns to release an egg—a predictable left-right-left-right pattern—is a widely held but incorrect assumption. While alternation occurs in some cycles, it is not a scheduled requirement for the reproductive system to function. The selection of which ovary will ovulate is determined by chance and a complex biological competition, not by a pre-set roster.
Studies show that one ovary can, and often does, ovulate multiple times in a row before the other takes over. Research suggests the right ovary may ovulate slightly more often than the left (approximately 55% vs. 45%). This slight preference may be due to anatomical differences, such as variations in the venous drainage systems.
How One Ovary is Chosen
The selection of the ovulating ovary is a dynamic process driven by a hormonal cascade beginning in the brain. At the start of the menstrual cycle, the pituitary gland releases Follicle-Stimulating Hormone (FSH), which prompts a group of follicles in both ovaries to begin developing. This process, known as follicle recruitment, involves approximately 10 to 20 immature eggs starting to mature.
These developing follicles compete with each other, each producing the hormone estrogen as it grows. Eventually, one follicle, known as the dominant follicle, outpaces the others in growth and estrogen production. The high levels of estrogen produced by this dominant follicle signal the pituitary gland to sharply increase its release of Luteinizing Hormone (LH).
This sudden surge of LH (the LH surge) is the hormonal trigger that causes the wall of the dominant follicle to rupture, releasing the mature egg. The ovary housing the follicle most responsive to FSH and the fastest-growing is the one that ultimately ovulates. This selection is based on a localized, competitive race for dominance, not on any mechanism that tracks which ovary ovulated last.
Why Having Two Ovaries Matters
The presence of two ovaries provides biological redundancy and resilience to the reproductive system. Each ovary contains hundreds of thousands of primordial follicles, representing the woman’s functional ovarian reserve. This redundancy means that if one ovary is compromised, damaged, or surgically removed, the remaining ovary can compensate for the loss.
When a woman has only one functioning ovary, studies show that the remaining ovary will often increase its output, suggesting a compensatory mechanism in oocyte yield. While a single ovary may yield fewer eggs per cycle than a pair, the ability to maintain fertility and hormone production is largely preserved. For instance, live birth rates following fertility treatments are often found to be equivalent between patients with one ovary and those with two.
The dual organ system ensures that the production of reproductive hormones, like estrogen and progesterone, continues uninterrupted. This supports the menstrual cycle and overall health even if one side becomes non-functional.