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Why does menstruation occur when an egg is not fertilized?

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Front view anatomical diagrams of the male and female reproductive systems, with labeled organs including penis, testis, prostate gland, uterus, ovary, and vagina. — human reproductive system Organs and structures of the male and female reproductive systems.

Menstruation occurs when an egg is not fertilized because of the hormonal changes that follow the failure of implantation.

During the menstrual cycle, the ovaries prepare the endometrium—the inner lining of the uterus—for a potential pregnancy by secreting estrogen and progesterone. If the egg is not fertilized, the corpus luteum (the structure in the ovary that produces these hormones) degenerates, causing a sharp drop in hormone levels.123

This withdrawal of estrogen and progesterone triggers a constriction of the uterine arteries, which interrupts the delivery of oxygen and nutrients to the endometrium. Consequently, the endometrial lining breaks down and is sloughed off, resulting in the vaginal bleeding characteristic of menstruation. If fertilization had occurred, the developing embryo would have signaled the corpus luteum to continue hormone production, thereby maintaining the endometrium to support the pregnancy.124