Ovulation is the brief, mid-cycle event where a mature egg is released from one of the ovaries. This process is highly regulated by a precise sequence of hormones and marks the only time in the menstrual cycle when pregnancy is naturally possible. A typical menstrual cycle is divided into two main phases by this event: the follicular phase, which prepares for ovulation, and the luteal phase, which occurs afterward.
The Hormonal Build-Up
The journey toward ovulation begins immediately after the menstrual period with the follicular phase, driven primarily by Follicle-Stimulating Hormone. This hormone, released by the pituitary gland in the brain, prompts several fluid-filled sacs in the ovaries, known as follicles, to begin maturing. Each follicle contains an immature egg, though typically only one follicle becomes “dominant” and continues to grow toward full maturity.
As the dominant follicle enlarges, the cells within it produce increasing quantities of the hormone Estrogen, specifically estradiol. This rising level of Estrogen performs two main functions in preparation for the egg’s release. First, it accelerates the maturation of the egg within the dominant follicle, ensuring it is ready for fertilization. Simultaneously, the Estrogen signals the uterus to begin thickening its inner lining, the endometrium, with blood and nutrients to create a supportive environment for a potential pregnancy.
The Ovulation Event
The peak concentration of Estrogen triggers the central event of the cycle. When Estrogen levels reach a sustained high threshold, they signal the pituitary gland to release a massive pulse of Luteinizing Hormone (LH), known as the LH surge. This surge is the direct trigger for ovulation, typically occurring about 24 to 36 hours after the LH level begins its rapid rise.
The Luteinizing Hormone surge causes the final, rapid maturation of the egg and weakens the wall of the dominant follicle. This pressure and enzymatic action cause the follicle to rupture, expelling the mature ovum from the ovary into the abdominal cavity. The egg is quickly captured by the fimbriae, the fringe-like projections at the end of the fallopian tube, where it begins its journey toward the uterus. Once released, the mature egg has a very short lifespan, surviving for only about 12 to 24 hours to be successfully fertilized.
Post-Ovulation Changes
Following the egg’s release, the reproductive system transitions into the luteal phase, characterized by a different hormonal environment. The ruptured follicle that housed the egg transforms into a temporary endocrine structure called the corpus luteum. This yellow-colored body is responsible for producing high levels of the hormone Progesterone, which becomes the dominant hormone of this phase.
Progesterone’s primary role is to complete the preparation of the uterine lining, making it receptive to a fertilized egg. It causes the endometrium to become thicker, more glandular, and highly vascularized, ready for implantation. Progesterone also suppresses the release of further follicle-stimulating and luteinizing hormones, preventing another ovulation from occurring in the same cycle.
Corpus Luteum Degradation
If fertilization and implantation do not happen, the corpus luteum naturally begins to degrade approximately 10 to 14 days after ovulation. This degradation causes Progesterone levels to fall sharply, which signals the body to shed the prepared uterine lining, resulting in the start of menstruation.
Recognizing Ovulation
The hormonal shifts that lead to and follow ovulation produce several observable physical changes that can be tracked to identify the fertile window. One of the most reliable indicators is a change in the consistency of cervical mucus, which becomes noticeably clear, slippery, and stretchy, often resembling raw egg whites. This change, driven by rising Estrogen, creates a hospitable environment that facilitates the movement and survival of sperm.
Another physical sign is a slight, sustained increase in Basal Body Temperature (BBT), the body’s lowest resting temperature. Progesterone, released by the corpus luteum after ovulation, causes the BBT to rise by approximately 0.5 to 1.0 degree Fahrenheit, confirming that the egg has been released. Charting this temperature shift over several days can indicate when ovulation has already passed.
Some individuals also experience a sensation known as Mittelschmerz, which is German for “middle pain.” This is a mild, one-sided twinge or cramping in the lower abdomen that occurs around the time of the egg’s release. Mittelschmerz is thought to be caused by the follicular rupture or the small amount of fluid and blood released from the follicle irritating the abdominal lining.