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Within the ovarian cycle, menstruation occurs once per cycle. It co-incides with the reduction in progesterone levels after the regression of the corpus luteum at the end of the cycle.
The inhibitory effect of progesterone on prostaglandin synthesis is lost. Consequently, there is a local increase in the levels of prostaglandin F2-alpha and E2. These mediators result in spiral artery constriction and dilatation. The superficial endometrium is shed and mixes with blood derived from the rupture of the spiral arteries.
The resultant mixture is the menses - menstrual flow - which traverse the cervix and vagina.