Ovulation is the monthly biological process where a mature egg is released from one of the two ovaries, ready for potential fertilization. Many people assume the ovaries operate like a scheduled system, strictly alternating the release of an egg (e.g., left-right-left-right). This common belief suggests a fixed, rotational schedule that is scientifically inaccurate. Understanding the true mechanism requires examining the underlying biological competition that takes place every cycle.
The Reality of Ovarian Selection
The idea that the left and right ovaries take strict turns ovulating is a widespread misunderstanding of reproductive biology. Scientific studies confirm that the selection of the ovulating ovary is not based on a predetermined, alternating schedule. Instead, the process is best described as a competitive event where the most hormonally responsive ovary “wins” the cycle. This internal competition means one ovary may release an egg several cycles in a row while the other remains dormant. The probability of either ovary ovulating is approximately 50/50, but this randomness does not enforce alternation. The pattern of ovulation is irregular and unpredictable, showing no consistent side-to-side switching over time.
Hormonal Drivers of Dominant Follicle Growth
The biological selection process begins early in the menstrual cycle with a rise in Follicle-Stimulating Hormone (FSH) secreted by the pituitary gland. FSH acts on both ovaries simultaneously, initiating the growth and development of a cohort of small follicles on each side. This simultaneous recruitment is the starting line for the competitive race that determines which ovary will release an egg.
As the follicles grow, they compete intensely for the available circulating FSH, and typically, only one follicle achieves dominance. This single structure is known as the dominant follicle, and the others in the cohort stop developing and regress. The dominant follicle becomes the primary source of the steroid hormone estrogen, which signals its readiness for release.
The rising level of estrogen produced by this growing follicle acts on the pituitary gland, causing a sharp increase in Luteinizing Hormone (LH). This rapid spike in LH, known as the LH surge, is the final trigger that causes the dominant follicle to rupture and release the mature egg. This selection process is internal to the ovaries and depends on which follicle, regardless of side, is the most robust and receptive to the circulating FSH signals.
When One Ovary Takes the Lead
While the standard process involves random, hormonal competition, certain physical circumstances can force one ovary to take the lead in a non-alternating pattern. If one ovary is surgically removed (oophorectomy), the remaining ovary fully compensates for the loss. The single remaining ovary will then reliably ovulate every cycle to maintain reproductive function.
Physical damage, such as scar tissue from endometriosis or infection, can impair the function of one ovary, making its follicles less responsive to FSH. In such cases, the healthier, undamaged ovary will consistently develop and release the dominant follicle. Temporary factors, such as a large, non-hormone-producing cyst, can also suppress one ovary’s function for a period. This suppression temporarily forces the other ovary to assume the responsibility of ovulation until the cyst resolves, illustrating the system’s adaptability.