Evaluation of Progesterone Resistance and the Role of Downregulation Protocols in Endometrial Preparation for Frozen Embryo Transfer in Patients with Recurrent Implantation Failure—A Retrospective Study

This study investigates the impact of progesterone resistance on endometrial receptivity and evaluates the efficacy of pituitary downregulation combined with hormone replacement therapy (HRT) in improving pregnancy outcomes following frozen embryo transfer (FET). A retrospective analysis was conducted on 147 patients with recurrent implantation failure (RIF) who underwent secretory-phase endometrial biopsy and subsequent FET treatments at the Reproductive Center of the First Affiliated Hospital of Guangxi Medical University between April 2019 and May 2022. Based on biopsy results and endometrial preparations, patients were categorized into four groups: secretory phase + non-downregulated protocol group (n=29), secretory phase + GnRH-a + HRT Protocol group (n=20), proliferative phase + non-downregulated protocol group (n=59), proliferative Phase + GnRH-a + HRT Protocol group (n=39). Endometrial preparation regimens and pregnancy outcomes during the first FET cycle after biopsy were compared. The results showed that there was no statistical difference in the outcome of assisted pregnancy between the different endometrial preparation protocols in the four groups. These findings suggest that the prioritization of down-regulation protocols may not be essential for FET in patients with RIF, even when endometrial biopsy during the implantation window reveals proliferative-phase characteristics.