Normal Menstrual Cycle

The normal menstrual cycle is 28 days and is divided into four phases: follicular, ovulation, luteal or secretory, and menses. The first 14 days are the follicular phase, during which the ovary matures an oocyte for ovulation, and the granulosa cells, lining the follicle, produce estrogen. This stimulates the endometrium to proliferate and thicken (Fig 98-1). In response to the rising estrogen levels, the pituitary gland secretes follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which stimulate the release of the mature oocyte. The residual follicular capsule forms the corpus luteum. During the luteal phase, the corpus luteum secretes estrogen and progesterone, which maintain the endometrium and make it more receptive to implantation. If fertilization and implantation occur, the developing embryo secretes human chorionic gonadotropin (hCG) into the bloodstream, signaling the corpus luteum to continue the production of progesterone and estrogen necessary to support early pregnancy. In the absence of hCG, the corpus luteum involutes, and estrogen and progesterone levels fall. Hormonal withdrawal causes vasoconstriction in the spiral arterioles of the endometrium. This leads to the final phase, or menses, when the ischemic endometrial lining becomes necrotic and sloughs. The vaginal effluvium contains blood, endometrial tissue, and fluid. The estimated amount of menstrual blood loss ranges from 25 to 60 mL. Judging the amount of bleeding in a menstrual cycle may be difficult. The average tampon or pad absorbs 20 to 30 mL of vaginal effluent, although the number of pads or tampons used is unreliable, as personal habits vary greatly among women. In a normal cycle, fibrinolysis occurs in the uterine cavity and the cervix. In women with heavy bleeding, there may be insufficient time for fibrinolysis, and blood clots may occur. Definitions of commonly used gynecologic terminology have been included in T§bIe.,...9.8.-1.

FIG. 98-1. Ihe hormonal, ovarian, endometrial, and basal body temperature changes and relationships throughout the normal menstrual cycle. (From Carr and Wilson,24 with permission.)

TABLE 98-1 Definitions
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