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Days 1-5: Menstruation
During menstruation, your body realizes that the egg from the previous cycle did not fertilize or implant. The estrogen and progesterone hormone levels will begin to drop causing the lining of the uterus, called the endometrium, to shed and bleed through the cervix and vagina. This bleeding lasts an average of five days and consists of blood, mucus, and tissue.
The first day of menstrual flow is considered the first day of the next menstrual cycle. During this menstrual phase, your pituitary gland produces a hormone called FSH (Follicle Stimulating Hormone). This hormone level rises and triggers the next phase in the menstrual cycle where your body begins to prepare for another reproductive cycle.