Biological feedback mechanisms involving the hypothalamus, anterior pituitary gland, ovaries, and endometrial lining of the uterus control the average 28-day menstrual cycle.8,9 The hypothalamus synthesizes gonadotropin-releasing hormone (GnRH) and secretes the hormone in a pulselike manner with varying frequencies throughout the menstrual cycle. GnRH stimulates the anterior pituitary to produce and release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH and LH act on the ovaries to produce estrogen and progesterone. Estrogen in turn acts on the hypothalamus and anterior pituitary, in a negative feedback manner, to stop FSH and LH secretion.
The menstrual cycle is divided into three phases: the follicular phase, ovulation, and the luteal phase (Fig. 45-2).8,9 By convention, the day bleeding begins is referred to as the first day (or day 1) of the menstrual cycle. Menstrual bleeding usually occurs between days 1 to 5 of the cycle. The follicular phase begins at the onset of menstruation (menstrual phase) and lasts approximately 10 to 14 days (Fig. 45-2). Also at the beginning of the follicular phase, several follicles begin to develop within the ovary. In the second half of the follicular phase, most of the developing follicles atrophy except for one “dominant” follicle that will develop further and produce increasing amounts of estrogen. Elevated estradiol levels cause a surge of both LH and FSH. This LH surge is responsible for final-stage growth and maturation of the follicle, ovulation, and the formation of the corpus luteum. Ovulation usually occurs 14 days before the last day of the cycle. After ovulation, the dominant follicle develops further and produces estrogen and progesterone in increasing amounts. In 90% of women, the luteal phase lasts 13 to 15 days and is the least variable part of the human reproductive cycle. During this progesterone-dominant phase, the corpus luteum produces both progesterone and estrogen. Progesterone prepares the endometrium for implantation of a fertilized ovum. If implantation does not occur, corpus luteum regression causes a decrease in the levels of estrogen and progesterone. When these hormone levels decrease, the endometrium cannot be maintained and is sloughed off (menstrual phase). Using the average 28-day cycle as an example, day 28 is the last day of the cycle and is the day before bleeding begins again for the next menstrual cycle.
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