Part of “CHAPTER 95 – THE NORMAL MENSTRUAL CYCLE AND THE CONTROL OF OVULATION“

A series of cyclic and closely related events involving the reproductive organs occur in normal, nonpregnant adult women at about monthly intervals between menarche, at approximately age 12 years, and menopause, at approximately age 51 years. These events constitute the menstrual cycle. During each normal menstrual cycle, an ovum matures, is ovulated, and enters the uterine lumen through the fallopian tubes. Steroids secreted by the ovaries effect endometrial changes, allowing implantation if the ovum is fertilized. In the absence of fertilization, ovarian secretion of progesterone and E₂ declines, the endometrium sloughs, and menstruation begins. The menstrual cycle requires the coordinated, functional interaction of the hypothalamus, the pituitary gland, and the ovaries (the hypothalamic–pituitary–ovarian axis) to produce associated changes in the target tissues of the reproductive tract (endometrium, cervix, and vagina), which then permit pregnancy and perpetuation of the species.8 Although the individual units of the hypothalamic–pituitary–ovarian axis are innervated, the mediators of the communication also include autocrine, paracrine, and hemocrine mechanisms.