What is included in menstrual discharge besides blood?

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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.

Menstrual discharge is not composed solely of blood. In addition to blood, the periodic discharge from the vagina includes various secretions and the disintegrating mucous membrane that previously lined the uterus, also known as the endometrium.1