The menstrual cycle is a monthly preparation for life that ends in shedding.

Science · Published 28 October 2026 Swipe more like this →

In short: While menstruation is often viewed as the beginning of a process, it is actually the conclusion of one. Every month, the body undergoes a complex hormonal dance to build a nursery; when no pregnancy occurs, that entire structure is dismantled and expelled.

The menstrual cycle is defined as the time from the first day of one period to the first day of the next. While 28 days is the standard average, biological variability allows for normal cycles ranging from 21 to 40 days (or up to 45 days during the first few years after puberty). This monthly sequence is driven by chemical messengers—hormones—released by the pituitary gland in the brain and the ovaries. These hormones coordinate a series of physical changes designed to create an environment where a fertilized egg can implant and grow.

The process involves three primary stages: menstruation, the proliferative phase, and the secretory phase. It begins with menses, where the uterine lining (endometrium) sheds because no pregnancy occurred. This is followed by the proliferative phase, typically lasting until day 14. During this time, a follicle in the ovary ripens to contain an ovum—a cell roughly 0.14 millimetres in diameter. As these follicles develop, they secrete estradiol, which causes the endometrium to thicken and become more vascular, creating a nutrient-rich environment.

The cycle reaches its peak at ovulation, usually around day 14, when a mature egg is released into the fallopian tubes. If sperm fertilizes the egg, it attaches to the thickened uterine lining, where a placenta develops to provide oxygen and nutrition to a fetus. If fertilization does not occur, the hormones shift, causing the endometrium to break down. This shedding—composed of blood, mucus, and tissue from the uterus, cervix, and vagina—is expelled as menstrual flow. The cycle repeats until menopause, typically around age 50, when ovarian function ceases.

Understanding these mechanics reveals that menstruation is not a random event but a highly regulated physiological response to hormonal fluctuations. Factors such as stress, diet, and strenuous exercise can influence the timing of these cycles, particularly during the first two years after a person reaches puberty.

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