A familiarity of the physiology of the menstrual cycle aids the emergency clinician in identifying and understanding causes of abnormal bleeding. As is often said, one must first know “normal” to know “abnormal.” The mean time between menstrual periods is 28 days (+/- 7 days); therefore, bleeding outside of this interval is abnormal. The first half of the menstrual cycle is known as the follicular phase. During this phase, the level of estrogen is gradually rising.
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