There are three main cycle regimens used for endometrial preparation for frozen embryo transfer（FET）: natural cycles (NC) with/without ovulation triggering, hormone replacement therapy cycles (HRT) in which the endometrium is artificially prepared by estrogen and progesterone hormones with/without a gonadotrophin releasing hormone agonist (GnRH-a) down regulation, and ovulation induced cycles (OI) in which follicular development is supported with increasing doses of gonadotrophin hormones and ovulation is induced. At present, there is still no sufficient evidence that which kind of FET cycle regimen to plan more advantage. The purpose of this study was to compare the pregnancy outcome of NC-FET to that of HRT-FET in women with regular cycles.

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.

Inclusion Criteria:

• Age forty years old or less
• Has regulation Menstruation (cycle 24-35 days) or previous data suggest that normal ovulation
• Frozen embryos number more than three

Exclusion Criteria:

• Has Chocolate cyst or adenomyosis of uterus
• Clear hydrosalpinx
• Uterine scar or intrauterine adhesion and endometrial thickness <7mm before ovulation
• recur thick endometrium（<7mm）
• repeated implantation failure（≥3 times）
• Cancel the cycle because of having no dominant follicle in the NC+FET before or the growth of endometrium not good in the HRT+FET before