What causes menstrual bleeding to begin?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram of the menstrual cycle showing hormone levels, follicle development, and endometrial changes across 28 days. It highlights menstruation, ovulation, and the follicular and luteal phases, with labeled curves for estrogen, progesterone, FSH, and LH. — menstruation Cyclical changes during a woman's normal ovulatory menstrual cycle.

Menstrual bleeding begins when the corpus luteum degenerates and ceases to produce the hormones estrogen and progesterone at the end of a cycle in which fertilization of the ovum has not occurred.123

The withdrawal of these hormones triggers a sudden spasm of the endometrial blood vessels, which interrupts the delivery of oxygen and nutrients to the uterine lining. Consequently, the tissue dies and is sloughed off, resulting in the vaginal bleeding characteristic of menstruation. This process marks the end of one menstrual cycle and the beginning of the next.13