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How does menstruation differ from the estrous cycle in other mammals?

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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.

The menstrual cycle is derived from the estrous cycle, but they differ primarily in how the uterus responds when pregnancy does not occur.1

In most mammals, the estrous cycle is characterized by a period of sexual receptivity known as estrus, or "heat," which typically coincides with ovulation. During this cycle, the uterine lining (endometrium) thickens to prepare for a potential pregnancy. If fertilization does not occur, the reproductive tract simply returns to a normal, quiescent state, often through a phase called metestrus.21

In contrast, the menstrual cycle—found in humans and other higher primates—involves the active shedding of the uterine lining. When progesterone levels drop due to the absence of fertilization, the blood vessels in the endometrium constrict, causing the tissue to disintegrate and be expelled from the body as menstruation. Additionally, while many mammals are restricted to specific breeding seasons, humans experience an unbroken series of cycles throughout the year.213