A Study:the Different Outcomes Between Natural Cycle and Hormone Replacement Cycle in FET

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.