Several studies have reported retrieval of more mature oocytes after GnRH agonist triggering compared to the number of oocytes retrieved after hCG. Among the possible advantages of GnRH agonist for final oocyte maturation is the simultaneous induction of an FSH surge. The role of the natural mid-cycle FSH surge is not fully clear. FSH was reported to induce LH receptor formation in luteinizing granulosa cells, and to promote oocyte nuclear maturation and cumulus expansion .