It is a physiological phenomenon that on the first day of menstruation, the menstrual corpus luteum of the ovary starts to atrophy and can still secrete a certain amount of progesterone, and the decrease of progesterone level in the body is not very obvious. With the complete atrophy of the corpus luteum, the level of progesterone in the body decreases rapidly, causing a large amount of endometrium in the secretion period to exfoliate and bleed, and the bleeding volume increases significantly, resulting in an increase in the menstrual flow. After 2-3 days, a large amount of endometrium is exfoliated and the remaining endometrium in the secretory phase is very little, and the endometrium exfoliated at a later stage decreases, the menstrual flow also decreases and gradually stops, which is the pattern in normal menstruation. In some women, the ovarian corpus luteum shrinks rapidly on the first day after entering menstruation, and the endometrium peels off in a large amount within a short time and more menstrual blood appears.