The sequence shown in Figure 4.1 begins with the secretion of GnRH from the hypothalamus, which stimulates the pituitary gland to produce FSH and LH, which in turn act on the ovaries, telling the ovarian follicles to complete the maturation of an ovum. During this period, a few ovarian follicles containing immature ova develop and mature under the stimulation of FSH and LH. Usually by day 14, only one follicle has become fully mature, and the ovum it contains is ready to be released. All other follicles that had begun maturing during this phase of the ovarian cycle degenerate as soon as ovulation has occurred.
Note that only one time interval is fairly constant in all females — the time from ovulation to the beginning of menstruation, which is almost always 14 to15 days. However, the time of ovulation is variable and difficult to predict accurately.