Recombinant gonadotropins are currently used for ovarian stimulation in women undergoing to IVF for infertility. Recently new flexible protocols have been introduced: the "single dose" and "multiple dose" protocol. The single dose protocol is performed just using Cetrorelix by the administration of a single injection of Cetrorelix 3 mg subcutaneous when the lead follicle is ≥ 14 mm. The multiple dose protocol consider the daily administration of Cetrorelix 0.25 mg subcutaneous or Ganirelix 0.25 mg subcutaneous when the lead follicle is ≥ 14 mm and until the realization of hCG criteriaThere are few data from letterature about the comparison of the efficacy in pitituary suppression and pregnancy outcome of Ganirelix and Cetrorelix. Objective: to compare the efficacy in pituitary gonadotropin suppression and IVF outcome of multiple dose flexible gonadotrophin-releasing hormone (GnRH) antagonist administration according to follicular size using daily injection of Cetrorelix or Ganirelix. Interventions : patients are randomly assigned in to Cetrorelix acetate group or Ganirelix acetate group. Multiple dose of Cetrorelix acetate (0.25 mg) or multiple dose of Ganirelix acetate (0.25mg) are administered when the lead follicle is ≥ 14 mm until hCG criteria are met.Oocyte maturation trigger is performed by the administration of subcutaneous r-hCG (250 μg) or of intramuscular hCG (10,000 IU). After 36 hour transvaginal oocyte retrieval is performed followed by ICSI and embrio transfer (72 hours later).