The in-vitro maturation culture medium consisted of G-IVF medium (Vitrolife Sweden AB, Goteborg, Sweden) supplemented with 10% human serum albumin (HSA) solution (Vitrolife), 25 mmol/L sodium pyruvate (Sigma), 75 IU/L recombinant FSH (Gonal-F; Merck Serono, The Netherlands), and 150 IU/L HCG (Ovidrel; Merck Serono, The Netherlands). Immature oocytes were cultured in a humidified atmosphere of 6% CO2, 5% O2, and 89% N2 at 37°C. The oocyte maturational status was evaluated after 15 h of in-vitro culture. Mature oocytes were identified if they extruded a polar body after 15 h of in-vitro culture and were then used for vitrification. Oocytes remaining immature after 15 h of in-vitro culture were considered incompetent for maturation and were discarded.