In order to develop, mature and ovulate, follicles need FSH. When this is completely lacking (hypogonadotropic hypogonadism) or does not attain a sufficient stimulatory threshold, owing to intrinsic inhibitors of action and/or a dysfunctional feedback mechanism (as in polycystic ovary syndrome (PCOS)), exogenous FSH can provide the stimulus for follicle development. At any one time, these anovulatory women have a limited cohort of follicles that are sensitive to FSH. In order for a follicle to grow, the threshold of its sensitivity needs to be reached and, in order to develop and mature, continued stimulation with FSH is required. Physiologically, in a normal ovulatory cycle, only one follicle will become dominant and ovulate, by a sophisticated mechanism that
provides it with great enough sensitivity to dwindling levels of FSH to maintain its development while, all around, smaller follicles do not achieve this level of sensitivity to FSH, are not stimulated and fall into atresia (Figure 2.7). Stimulation with FSH for ovulation induction should follow similar principles. Thus, enough FSH should be given exogenously to reach a threshold sufficient to initiate growth and development of a number of follicles, but overstepping the threshold sufficiently to maintain the growth of one follicle, and certainly not more than three. This principle should be separated from that of controlled ovarian hyperstimulation, in which larger doses of FSH are administered with the purpose of producing more follicles and oocytes in sufficient numbers for in vitro fertilization.