THE FOLLICLE-STIMULATING-HORMONE THRESHOLD LEVEL FOR FOLLICLE MATURATION IN SUPEROVULATED CYCLES

Objective: To compare the FSH threshold level for follicle growth up to the preovulatory stage between superovulated and natural cycles. Design: Normally cycling women were investigated during an untreated spontaneous cycle (control) and three cycles treated with FSH. Setting: Infertility clinic in a university hospital. Patients: Thirteen normally ovulating women with unexplained infertility. Interventions: Follicle-stimulating hormone was injected at the single dose of six ampules on cycle day 2 without extra FSH (FSH + 0 cycle) or with extra FSH from cycle day 4 onward at the daily dose of either one ampule (FSH + 1 cycle) or two ampules (FSH + 2 cycle). Main Outcome Measures: Serum FSH and E(2) during the follicular phase. Number of follicles and growth rate by ultrasound. Results: Multiple follicular development occurred in all FSH treated cycles. In these cycles the growth rate of the first (dominant) follicle was similar to that in the control cycles. In the presence of physiological concentrations of FSH (control and FSH + 0 cycles), only one follicle was >15 mm at midcycle. When serum FSH values increased (FSK + 1 and FSH + 2 cycles), the growth rate of the second and third in order follicles and subsequently the number of the preovulatory follicles (>15 mm) increased in proportion to the FSH increment. Conclusions: Follicles recruited by exogenous FSH in the early follicular phase of the cycle require supraphysiological concentrations of FSH throughout the follicular phase to become preovulatory. The extra FSH acts through an increase in the growth rate of these follicles.