Endometrial Preparation Useful to Embryo Implantation

Endometrial preparation is crucial in in vitro fertilization (IVF) protocols to create optimal conditions for embryo implantation and pregnancy. Successful embryo nidation requires a receptive endometrium, which is influenced by factors such as endometrial thickness, hormonal environment, autocrine and paracrine growth factors, and the presence of resident and non-resident stem cells within the endometrium basalis. Synchronization between endometrial and embryonic development is also essential. Endometrial preparation, whether achieved through spontaneous cycles, natural cycles with minimal support, ovulation induction, or hormone replacement therapy (HRT), results from a combination of biochemical and biophysical conditions, each independent of the others. These conditions are difficult to pinpoint within a fixed period (Window of Implantation, WOI) using any specific examination due to the variability in estrogen and progesterone production rates, their metabolic clearance rates (MCR) when administered exogenously, and their metabolism when given systemically, topically, or orally. These variations also occur in different phenotypes and over different durations. Moreover, the synchronization of endometrial preparation with incomplete decidualization and the uncertain duration of the implantation window pose challenges. It is also unknown if there are compliance mechanisms similar to diapause seen in other species. This review aims to outline a clinical approach that, while not an absolute clinical recommendation due to the lack of definitive certainties, assists in understanding complementary phenomena and provides a useful orientation for clinical practice.