The menstrual cycle is a series of natural changes in hormone  | International Up-to-date Clinical Channel, IUCC

The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls production and release of eggs and the cyclic release of estrogen and progesterone. The uterine cycle governs the preparation and maintenance of the lining of the uterus to receive a fertilized egg.
Naturally occurring hormones drive the cycles; the cyclical rise and fall of the follicle stimulating hormone prompts the production and growth of oocytes. Estrogen stimulates the uterus lining to thicken to accommodate an embryo should fertilization occur. The blood supply of the thickened lining (endometrium) provides nutrients to a successfully implanted embryo. If implantation does not occur, the lining breaks down and blood is released. Triggered by falling progesterone levels, menstruation is the cyclical shedding of the lining, and is a sign that pregnancy has not occurred.

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