The process of ovulation involves the release of a mature egg from an ovary, occurring approximately once per menstrual cycle. A common question is whether both ovaries participate simultaneously. The answer is generally no; standard human reproductive physiology is designed to release a single egg from only one of the two ovaries during a given cycle. This single-egg release is the body’s usual mechanism to prepare for potential pregnancy.
The Standard: Unilateral Ovulation
In a typical menstrual cycle, the body selects and matures one follicle, which contains the egg, for release in a process known as unilateral ovulation. This mechanism ensures that only one oocyte is available for fertilization, limiting the possibility of a multiple pregnancy. Although it might seem logical that the ovaries would alternate the task of releasing an egg, this is not the case for most cycles.
The selection of which ovary will ovulate is primarily a random event rather than a predictable, alternating pattern. Studies show that the side of ovulation in one cycle does not strongly influence the side of ovulation in the next. A woman might ovulate from the same ovary for several consecutive months before the other takes over.
Some evidence suggests a slight physiological preference, with the right ovary sometimes ovulating marginally more often than the left, potentially due to differences in blood flow. Regardless of the side, the selection process isolates a single dominant follicle for release. The body’s hormonal signals reinforce the goal of maturing just one egg.
The Hormonal Mechanism of Follicle Selection
The reason only one ovary releases an egg is a precise hormonal feedback system. The process begins early in the cycle with a rise in Follicle-Stimulating Hormone (FSH), which prompts a cohort of small follicles in both ovaries to begin growing. As these follicles enlarge, they start producing estrogen, signaling back to the brain.
The rising estrogen levels act as a negative feedback signal, causing the pituitary gland to reduce its production of FSH. This drop in systemic FSH creates a competitive environment among the growing follicles. The follicle slightly ahead in development, known as the dominant follicle, has already acquired a greater number of FSH receptors on its surface.
This greater receptor count allows the dominant follicle to continue growing and maturing even as FSH levels decline. The smaller, less-developed follicles cannot survive the reduced FSH environment and undergo programmed cell death called atresia. This hormonal pruning ensures that only the single, most robust follicle reaches full maturity, preventing simultaneous ovulation.
The dominant follicle then releases a surge of estrogen, which triggers the Luteinizing Hormone (LH) surge. This LH surge is the final step that causes the follicle to rupture and release the mature egg.
Hyperovulation and the Basis of Fraternal Twins
The exception to unilateral ovulation is hyperovulation, where more than one egg is released during the same menstrual cycle. This deviation occurs when the hormonal mechanism that selects a single dominant follicle fails or is overridden. The multiple eggs released can originate from the same ovary or from one egg being released by each ovary.
Hyperovulation is the physiological basis for the conception of dizygotic, or fraternal, twins. If two eggs are released and successfully fertilized by two separate sperm, the result is two genetically distinct embryos. Several factors can increase the likelihood of hyperovulation, including a genetic predisposition in the maternal line.
Maternal age is also a factor, as women approaching the later end of their reproductive years may experience hormonal fluctuations leading to higher levels of FSH. This can stimulate more than one follicle to mature. Additionally, many fertility treatments intentionally induce hyperovulation to increase the chances of a successful pregnancy. This controlled stimulation overrides the body’s natural preference for single-egg release.