Cryopreserved embryo transfer: endometrial preparation and timing.

The objective of this article was to review and synthesize information from the scientific literature pertaining to the preparation of endometrium for cryopreserved embryo transfer. This article is a critical review of selected scientific literature, synthesis, and formulation of opinion. Estrogen and progesterone are necessary and sufficient to induce endometrial receptivity in cryopreserved embryo transfer cycles. A variety of regimens have been described including natural cycles using endogenous ovarian hormones and artificial or programmed cycles with exogenously administered steroid hormones. To achieve optimal synchrony between embryo and endometrium, the timing of progesterone administration needs to be adjusted to the developmental stage of the thawed embryos. There is currently no evidence that any single regimen or adjuvant substance results in superior outcomes in cryopreserved embryo transfer cycles, although timing of progesterone administration does matter. Although no single regimen of endometrial preparation for cryopreserved embryo transfer has been proven superior to the others, the relative convenience and ease of use do vary, depending on the route of administration chosen and any adjuvant added to the cycle.