The Normal Menstrual Cycle

Figure 4

The ovary produces 95% of the circulating estrogen in the form of estradiol.(13,14) The second major estrogen is estrone, which is derived from the metabolism of estradiol and the conversion of androstenedione from body fat. A review of the endocrinology of the normal menstrual cycle, as outlined in Rousseau,(15) is warranted.

The normal menstrual cycle is mediated by a complex interplay of hormones modulated by the hypothalamic-pituitary-ovarian axis. Under the influence of gonadotropin-releasing hormone (GnRH) from the hypothalamus, the pituitary gland produces follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which in turn stimulate the production of estrogen and progesterone by the ovary.

These hormones are not secreted at constant rates throughout the monthly cycle. Instead, they interact in a feedback mechanism primarily involving FSH, LH, estrogen, and progesterone. Hormone levels vary drastically during different times of the cycle. Physiologically, the delicate sequential balance of hormone production allows maturation of the ovarian follicle and release of an ovum, as well as preparation of the endometrium for implantation of a fertilized ovum.

The normal menstrual cycle represents an intricate process that involves 3 successive phases: follicular (or proliferative), periovulatory, and luteal (or secretory). Menses coincides with the beginning of the follicular phase (Figure 4). At the onset of menses, estrogen and progesterone levels in the blood are low. This signals the hypothalamic-pituitary connection to release large amounts of FSH and LH. The FSH initiates the process of follicular maturation, and the follicle increases the production of estrogen to stimulate new endometrial growth. At the end of the follicular phase, the endometrium has thickened threefold, and a primordial follicle has matured in preparation for the release of an ovum.

During preovulation, increased estrogen from the mature follicle triggers a sharp increase in LH, causing release of the ovum. After this release, LH continues to stimulate the residual ovarian follicle, transforming it into the corpus luteum.

Figure 6

The secretory or luteal phase involves increased progesterone secretion from the corpus luteum to support and enrich the thickened endothelium. If fertilization of the ovum does not occur, estrogen and progesterone levels drop and menses takes place (Figure 5).

Figure 5