Physiology of Normal Menstrual Cycle

Understanding the mechanisms of the normal menstrual cycle lays the foundation for understanding the path physiology of anovulatory bleeding and its treatment.

Menstrual cycle resulting in ovulation is the result of a complex interaction between the various organs. A normal menstrual cycle occurs every 21-35 days with menstruation for 2-7 days. The average blood loss is 35-150 ml, with usually no more than 2 heavy days.

During the normal menstrual cycle, the first day corresponds to the first day of menses. The menstrual phase usually lasts 4 days and involves disintegration and soughing of the functionalis layer of the endometrium. The Prolifeeration (Follicular) phase extends from the day 5 to day 14 of the typical cycle. It is marked by endometrial proliferation bought on by estrogen stimulation.

The estrogen is produced by the developing ovarian follicles under the influence of FSH. The phase ends as estrogen produced by the developing ovarian follicles under the influence of FSH.

The phase ends as estrogen production peaks, triggering the FSH and LH surge. Repture of the ovarian follicle follows with release of the ovum (Ovulation). The Secretary (Luteal) phase is marked by the production of progesterone and less potent estrogen by the corpus luteum. It extends from the day 15 to day 28 of the typical cycle. The functionalis layer of the endometrium increases in thickness and stroma becomes oedematous. If fertilization does not occur the estrogen and progesterone feedback to the hypothalamus, and FSH & LH production falls. The spiral arteries become coiled and have decreased flow. At the end of the cycle, they alternately contract and relax, causing a breakdown of the functionalis layer and menses to begin.

Pathway of action of hormones for normal menstrual function. A menstrual cycle of fewer than 21 days or more 35 days or a menstrual flow of fewer than two days  or more than seven days is considered abnormal.