Introduction: Frozen 2: an update on cryopreserved embryo transfer in the era of vitrification

Cryopreservation of reproductive tissues has recently reached an unprecedented level of efficiency. Whereas sperm cryopreservation had been successfully practiced for many years, until relatively recently, cryopreservation of embryos resulted in the loss of many potential implantations, when compared to fresh transfer. Embryo survival rates of approximately 80% with slow freezing methods were common, and the subsequent transfer of frozen-thawed embryos was associated with decreased implantation rates, despite good endometrial preparation techniques. The uptake of rapid freezing techniques, often called vitrification, has changed this calculus. New debates now address the question of whether all embryos should be cryopreserved, rather than simply those that cannot be safely transferred in a fresh cycle. The following series of review articles addresses the technology of cryobiology, the optimal preparation of the endometrium for cryopreserved embryo transfer, and the question of whether the presence of the corpus luteum at the time of embryo implantation may influence the subsequent development of hypertensive disorders of pregnancy.