All women underwent controlled ovarian stimulation (COS) with a GnRH antagonist fixed regimen. Bilateral antral follicles (10mm) were counted by transvaginal ultrasonography on the second day of the menstrual cycle, and women started COS treatment with gonadotrophins (Gonal-F, Merck Serono Europe Ltd or Puregon, N. V. Organon). The levels of serum progesterone (SP) were measured using an automated electrochemiluminescence immunoassay (Roche Diagnostics Elecsys Cortisol II assays and COBAS E801), and values were expressed in ng/ml. At our center, the starting dose of gonadotrophins was 150 IU/day for women aged ≤ 34 years, with BMI <24 kg/m2, 6≤AFC<15, and the dosage would be increased if the woman was older (age ≥ 35 years), heavier (BMI ≥ 24 kg/m2), or had poorer ovarian reserve AFC<6 or basal FSH>10 IU/L or AMH <1 ng/ml. Conversely, if the woman is lean (BMI < 19 kg/m2) or has a good ovarian reserve (AFC≥ 15 or AMH≥ 4 ng/ml), the dosage will be reduced. Dose adjustments were determined by the physician based on individual clinical experiences. GnRH antagonists (Cetrorelix, BaxterorGanirelix, N.V.Organon) were given daily starting on day 5 or 6 of stimulation. Human chorionic gonadotrophin (Chorionic Gonadotrophin for Injection, Livzon) was injected once there were at least three follicles >17 mm in diameter or at least two follicles >18 mm in diameter. Oocyte retrieval was performed under ultrasound guidance 34-36 hours after triggering.
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