Is the endometrium ready for implantation? The importance of endometrial receptivity and the implantation window in RIF

Abstract

Recurrent Implantation Failure (RIF) is a multifactorial diagnosis encountered in assisted reproductive technology practice; its development is influenced not only by embryonic characteristics but also by the structural and functional status of the uterus and endometrium. This work analyzes endometrial receptivity and the “Window of Implantation” (WOI), which are critical factors for successful embryo implantation, and their potential role in RIF. It also discusses the transition of the endometrium to a receptive state under hormonal influence, the temporal synchronization between the embryo and the endometrium, and the individual nature of the WOI. Furthermore, the article examines methods for assessing the endometrium, including ultrasonography, histology, and molecular techniques, with a specific focus on endometrial receptivity tests, chronic endometritis, and the diagnostic significance of the CD138 immunohistochemical marker. Based on current scientific evidence, it is noted that endometrial receptivity and the WOI are not the sole causes of RIF and that there is insufficient evidence to support the routinal use of receptivity tests for all patients. In RIF, it is also advisable to assess endometrial factors using an individualized, complex approach that takes into account the clinical context, embryonic characteristics, and uterine status.