The cyclic production of estrogen and progesterone during the ovarian cycle induces a corresponding cycle of endometrial proliferation and development
(Fig. 1, bottom panel). This endometrial cycle includes a proliferative phase lasting about 11 days, a secretory phase lasting about 12 days, and a desquamative or menstrual phase of about five days during which menstruation ensues.
During the follicular phase of the ovarian cycle, the endometrium proliferates and increases in thickness under the influence of estradiol. This marks the proliferative phase of the endometrial cycle. During the luteal phase of the ovarian cycle, progesterone stimulates further endometrial cell proliferation, differentiation, and secretory development to support implantation and to nourish the developing conceptus. This is the secretory phase of the endometrial cycle. Progesterone also promotes secretory changes in the lining of the fallopian tubes that will support the fertilized egg as it travels down the fallopian tube prior to implantation.
If fertilization occurs, human chorionic gonadotrophin produced by the developing placenta maintains the corpus luteum, thereby sustaining progesterone secretion. If fertilization does not occur, the corpus luteum is lost, causing a sudden drop in progesterone. This triggers endometrial desquamation, in an ordely fashion, resulting in menstruation.
Total menstrual blood loss varies from cycle to cycle, among individual women, and at different stages of reproductive life. Average menstrual blood loss is about 50 to 60 mL, but may vary from 10 mL to over 100 mL (4,5). Chronic menstrual blood loss greater than 80 mL results in anemia. A loss of 60 to 80 mL has served to define "heavy flow" (4,5), although the clinical utility of this definition is negligible because most women cannot accurately assess their flow level (4).
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