Medication Protocol
All the subjects received a standardized long-term protocol in mid-luteal phase or gonadotropin-releasing–hormone (GnRH) antagonist regimen for ovarian stimulation. For long-term protocol, GnRH agonist (GnRHa) was used to downregulate the function of the pituitary gland on day 21 of the previous menstrual cycle. A dose of recombinant FSH ranging from 75 to 225 IU was initiated after down-regulation. As for GnRH antagonist regimen, recombinant FSH at a dose of 75 to 225 IU was administered on day 3 of the menstrual cycle. GnRH antagonist at a dose of 0.25mg daily was initiated when at least one follicle was 12 mm or more in the mean diameter. For all two protocols, urinary human chorionic gonadotropin (hCG) at a dose of 4000 to 8000IU was intramuscularly injected when at least two dominant follicles reached 18–20 mm in diameter and oocyte retrieval was performed 36–48 h later. 50ml follicular fluid was collected after oocyte retrieval.