The menstrual cycle prepares the female body for potential pregnancy each month. This cycle is controlled by communication between the brain and the ovaries, the two glands located on either side of the uterus. The ovaries produce eggs (ova) and secrete reproductive hormones like estrogen and progesterone. The central event is ovulation, the release of a mature egg from one of the ovaries.
The Mechanism of Ovulation
The process of ovulation begins with the follicular phase, initiated by hormones released from the pituitary gland in the brain. Follicle-Stimulating Hormone (FSH) travels through the bloodstream to the ovaries, where it prompts the growth of a cohort of small, fluid-filled sacs called follicles, each containing an immature egg. As these follicles grow, they begin to produce the hormone estrogen, which causes the lining of the uterus to thicken in preparation for a potential pregnancy.
Typically, only one follicle becomes the dominant one, continuing to grow while the others stop developing and regress. The increasing levels of estrogen produced by this dominant follicle signal the pituitary gland to release a large amount of Luteinizing Hormone (LH), known as the LH surge. This surge is the direct trigger for ovulation, causing the dominant follicle to rupture and release its mature egg from the ovarian surface. This release usually occurs about 28 to 36 hours after the onset of the LH surge, marking the end of the follicular phase.
The Reality of Ovarian Selection
The idea that the ovaries strictly alternate releasing an egg each month is a widely held but inaccurate belief. While ovulation from opposite ovaries is common, the process is not mandated to switch with every cycle. Instead, the selection of which ovary will ovulate is functional, often appearing random, meaning one ovary can ovulate multiple times in a row.
The selection is best described as a “race” between developing follicles in both ovaries for the dominant position. Once a dominant follicle is chosen, the resulting ovulation will occur from that ovary. Although one histological study found that ovulation alternated in approximately 87.6% of the cycles examined, this still suggests that alternation is a strong tendency rather than a guaranteed, mechanical switch.
The general pattern appears to be a random selection that favors the ovary which did not ovulate in the previous cycle. Studies have indicated a slight preference for the right ovary, which is responsible for approximately 55% of ovulations compared to the left ovary’s 45%. This preference highlights that ovarian selection is governed by complex physiological factors, not a simple biological timer.
Factors Affecting Which Ovary Dominates
The slight preference for the right ovary is likely due to anatomical differences in blood supply and venous drainage. A more robust vascularization provides the developing follicles in that ovary with a greater supply of the necessary hormones and nutrients for growth and maturation. This localized difference in blood flow can give the right ovary a subtle competitive advantage in the race to develop the dominant follicle.
The activity of the previous cycle also plays a significant role in determining the side of the next ovulation. After an egg is released, the empty follicle transforms into the corpus luteum, which produces high levels of progesterone. Research suggests that the next dominant follicle is more likely to occur in the ovary that has lower progesterone levels during the early follicular phase. Since the ovulating ovary contains the active corpus luteum, the other ovary, with its lower local progesterone concentration, is often favored for the next cycle, explaining the common pattern of alternation.
The Egg’s Journey to the Uterus
Once the mature egg is released from the ovary, it must be captured and transported by the adjacent fallopian tube. The fallopian tube is not directly attached to the ovary; instead, its fringed, finger-like projections called fimbriae actively sweep over the ovary to collect the released egg. Cilia lining the inside of the tube then gently move the egg toward the uterus, a journey that can take several days.
The egg remains viable for fertilization for a short window, typically about 12 to 24 hours after its release. If sperm are present, fertilization most often takes place within the fallopian tube. The corpus luteum continues to secrete progesterone, which prepares the uterine lining for the implantation of a fertilized egg. If fertilization does not occur, the egg disintegrates, the corpus luteum shrinks, and the uterine lining is shed during menstruation, beginning the cycle anew.