What is the physiology of a menstrual cycle?
The normal menstrual cycle is a tightly coordinated cycle of stimulatory and inhibitory effects that results in the release of a single mature oocyte from a pool of hundreds of thousands of primordial oocytes. A variety of factors contribute to the regulation of this process, including hormones and paracrine and autocrine factors that are still being identified. The cyclic changes in the major pituitary and gonadal hormones are illustrated in the figures (figure 1 and figure 2).
The physiology of the normal menstrual cycle will be discussed here. Detection of ovulation and ultrasound evaluation of the menstrual cycle are reviewed separately. (See "Ultrasound evaluation of the normal menstrual cycle" and "Evaluation of the menstrual cycle and timing of ovulation".)
PHASES AND DURATION OF THE MENSTRUAL CYCLE
By convention, the first day of menses represents the first day of the cycle (day 1). The cycle is then divided into two phases: follicular and luteal.
●The follicular phase begins with the onset of menses and ends on the day before the luteinizing hormone (LH) surge.
●The luteal phase begins on the day of the LH surge and ends at the onset of the next menses.