A Randomized Controlled Study:the Different Outcomes Between Natural Cycle and Hormon Replacement Cycle in FET

This study is a prospective randomized controlled trial. Patients with regular cycles undergoing FET in reproductive medicine renter of Sun Yat-sen Memorial Hospital will be recruited，who should not be elder than 40 and had more than 3 frozen embryos. They will be randomized to receive either the NC-FET cycle (group NC-A) or the HRT-FET cycle (group HRT-B). In group NC-A, the ovulation is monitored spontaneously or induced by human chorionic gonadotropin(HCG) when the dominant follicle is larger than 18mm without luteinizing hormone surge(LH). Transfer of thawed embryos will be performed 4 days after LH /HCG administration or 3 days after ovulation is observed. In group HRT-B, oral estradiol, 2 mg, once daily, is introduced on cycle day 3 with an increasing doses protocol. If the endometrial thickness is greater than 7mm, progesterone 40-60 mg in oil will be administered via intramuscular injection. Transfer of thawed embryos will be performed 3 days later. This study was approved by the reproductive medicine ethics committee of Sun Yet-sen Memorial Hospital.