The Normal Menstrual Cycle and the Control of Ovulation

Menstruation is the cyclic, orderly sloughing of the uterine lining, in response to the interactions of hormones produced by the hypothalamus, pituitary, and ovaries. The menstrual cycle may be divided into two phases: (1) follicular or proliferative phase, and (2) the luteal or secretory phase. The length of a menstrual cycle is the number of days between the first day of menstrual bleeding of one cycle to the onset of menses of the next cycle. The median duration of a menstrual cycle is 28 days with most cycle lengths between 25 to 30 days (1-3. Patients who experience menstrual cycles that occur at intervals less than 21 days are termed polymenorrheic, while patients who experience prolonged menstrual cycles greater than 35 days, are termed oligomenorrheic. The typical volume of blood lost during menstruation is approximately 30 mL (4). Any amount greater than 80 mL is considered abnormal (4). The menstrual cycle is typically most irregular around the extremes of reproductive life (menarche and menopause) due to anovulation and inadequate follicular development (5-7). The luteal phase of the cycle is relatively constant in all women, with a duration of 14 days. The variability of cycle length is usually derived from varying lengths of the follicular phase of the cycle, which can range from 10 to 16 days. For complete coverage of this and related topics, please visit www.endotext.org.