Introduction: Endometrial receptivity: evaluation, induction and inhibition.

Endometrial receptivity is an essential component of the complex process of embryo implantation. Its existence is inferred from the observation that not all embryo transfers result in pregnancy. The endometrium is a unique tissue which undergoes dramatic and rapid changes throughout the menstrual cycle. There appears to be a window of implantation, a time of optimal endometrial receptivity, when embryos are most likely to implant. The assessment of the timing and duration of this window of implantation has been a topic of interest and debate since the 1950s. The existence of the window of implantation led to the development of cycles in which endometrial receptivity is induced with exogenous E2 and P. These cycles are essential to third party parenting and frozen embryo transfers and have therefore become a common part of the practice of assisted reproduction.