In the female reproductive cycle menstrual flow occurs when the concentrations of

estrogen and progesterone are low

In women, menstrual flow occurs as a result of the actions of some hormone between the pituitary gland in the brain and the ovaries. Female sex hormones prepare the body for fertilization every month and without fertilization there is menstrual flow. Menstruation is activated by reduced levels of the hormones estrogen and progesterone at the end of the previous menstrual flow.

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