Endometrial preparation and embryo transfer
Three main types of endometrial preparation protocols were applied: the natural cycle (NC), hormone replacement treatment (HRT) cycle with or without GnRH downregulation. In NC cycles, growth of follicles were monitored under
transvaginal ultrasonography from cycle day 9 to 11. LH and estradiol were measured every 3 days after the diameter of leading follicle ≥ 1.4 mm. Intramuscular progesterone injections (40 mg/day) were started at the day of ovulation and FBT was scheduled on the 5th day after ovulation. HRT was performed with 6-8 mg oral estradiol valerate daily from cycle day 1 to day 14. Progesterone injection (60-100 mg) was administrated as soon as the endometrial thickness reached 7-8 mm and then FBT was scheduled after 5 days of progesterone therapy. As for HRT with GnRH- downregulation cycles, GnRH agonist was initiated on day 1 of the menstrual cycle. And on day 1 of subsequent menstruation, estrogen stimulation was started as HRT cycles without GnRH agonist. Embryo transfer was performed with a Guardia Access Embryo Transfer catheter (K-JETS-7019-SIVF, Cook, IN, USA) under transabdominal ultrasound guidance. Luteal support continued until 10 weeks of pregnancy.