menstrual cycle, the recurring cycle of change in the endometrium during which the decidual layer of the endometrium is shed, then regrows, proliferates, is maintained for several days, and is shed again at menstruation. The average length of the cycle from the first day of bleeding of one cycle to the first of another is 28 days. The duration and character vary greatly among women. Normally, menses occur at a frequency of 24 to 38 days, with frequent menses considered to be less than 24 days and infrequent menses greater than 38 days. Cycles begin at menarche and end with menopause. The uterine phases of the cycle are the menstrual phase, proliferative phase, and secretory phase. See also oogenesis, menses.
