Treatment
All patients received a standard ovarian stimulation protocol (antagonist or long protocols), oocyte retrieval, fertilisation, embryo cultured and cryopreservedin vitro , and a luteal phase support protocol after embryo transfer (ET), according to a routine method. All patients who underwent FET in this study were found unsuitable for undergoing a fresh embryo transfer or did not get a viable neonate after undergoing a fresh embryo transfer. The outpatient physician chose the most appropriate protocol based on clinical indications of the prepared endometrium, which mainly included a natural cycle, hormonal replacement therapy cycle and ovulation induction cycle. Frozen blastocysts were thawed and transferred, and subsequently provided with luteal phase support according to different endometrial preparation programs.