RESUMO
The success rate of several advanced basic and clinical techniques in the field of mammalian biotechnology, including cloning, pre-implantation genetic diagnosis, and assisted reproductive techniques [ART] depends mainly on the success rate of pregnancy following in vitro fertilization-embryo transfer [IVF-ET]. The techniques used in ART have advanced considerably since the first in vitro fertilization birth in 1978. However, despite these advances, pregnancy rates are still relatively low and have not increased significantly in the last decade. Based on the facts that embryo implantation is considered as the last barrier in ART and that inadequate endometrial receptivity is responsible for approximately two-thirds of implantation failures, intensive research work has been performed to understand the physiology, regulation, and the clinical assessments of the endometrial receptivity to improve the success rate of IVF-ET. This and the ongoing reviews tend to cover the different aspects of the endometrial receptivity mainly in human model. The present part of this series primarily concerns with biochemical and molecular events in the endometrium coordinated within its receptivity period termed as the window of implantation. Successive sections will deal with its ultrastructural changes, biomarkers, clinical assessments and regulators of endometrium within the window of implantation