Ovulation is a central event in the reproductive cycle, marking the release of a mature egg prepared for potential fertilization. A common question regarding the menstrual cycle is whether the left and right ovaries strictly take turns each month. Although the body has two ovaries capable of this function, the mechanism determining which one releases the egg is not based on a predetermined schedule. Understanding the mechanics of ovulation clarifies the true nature of this monthly process.
What Ovulation Is
Ovulation is the phase of the menstrual cycle when a mature egg, or oocyte, is expelled from the ovary. This event typically occurs around the middle of a person’s cycle, generally about 14 days before the start of the next menstrual period. The entire process is a necessary step in preparing the body for potential pregnancy.
The egg is housed within a fluid-filled sac called a follicle, which grows and matures inside the ovary during the first half of the cycle. Once the follicle reaches its full size, a surge of hormones triggers the ovarian wall to weaken and rupture. This action releases the mature egg into the pelvic cavity, where it is quickly captured by the feathery ends of the nearby fallopian tube.
The released egg then travels down the fallopian tube, awaiting possible encounter with sperm. The structure that remains in the ovary after the egg’s expulsion, the corpus luteum, immediately begins secreting hormones to support an early pregnancy. If the egg is not fertilized, the corpus luteum dissolves, signaling the body to begin menstruation.
Is Ovulation Random or Alternating?
The popular idea that the ovaries strictly alternate which side ovulates each month is a long-standing misconception. Studies tracking ovarian activity show that the selection process for the ovulating ovary is largely random and independent for each cycle. There is no biological clock or switch mechanism that mandates a left-right pattern.
The chance of ovulating from the same ovary two months in a row is approximately the same as the chance of switching sides, similar to flipping a coin. For example, in one study of normally fertile women, the ovulation side alternated in about 51% of succeeding cycles, which is statistically no different from a purely random distribution. This randomness means a person might ovulate from the right ovary for several cycles before the left ovary takes over.
Interestingly, some research suggests a slight tendency for the right ovary to ovulate more frequently than the left, indicating the right side may be responsible for about 55% of ovulations. This subtle preference may be due to minor anatomical differences, such as variations in blood supply. However, even with this slight bias, the cycle-to-cycle selection remains largely unpredictable rather than strictly alternating.
The Process of Follicle Selection
The randomness of the ovulating side is rooted in the complex process of follicle selection, which is controlled by hormonal signals rather than a pre-set anatomical schedule. At the beginning of the menstrual cycle, a group of follicles in both ovaries begin to grow in response to the pituitary hormone, Follicle-Stimulating Hormone (FSH). This phase is essentially a race between the follicles.
As the follicular phase progresses, one follicle naturally outpaces the others, becoming the “dominant follicle.” This dominant follicle begins producing large amounts of estrogen, which causes the pituitary gland to reduce its output of FSH. The resulting drop in FSH causes the smaller, less developed follicles to stop growing and degenerate.
The follicle that is the most responsive to the initial FSH signal is the one that survives this selection process and continues to mature. The final trigger for ovulation is a surge of Luteinizing Hormone (LH), which is released once the dominant follicle’s estrogen production reaches a certain threshold. This LH surge causes the dominant follicle to rupture, completing the process of ovulation.