Menstruation is the result of complex changes in the endometrium under hormonal control and the result of failure to conceive. For regular menstruation to occur, the hypothalamus, pituitary, ovary and uterus must be intact. In a normal menstrual cycle, a follicle must develop, rupture and luteinize. A normal menstrual cycle varies in duration, 24–32 days being the average time from the first day of bleeding to the first day of the next cycle.
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