Recurrent implantation failure: a registry-corroborated, age-stratified definition and pragmatic five-tiered management algorithm.

Recurrent implantation failure (RIF) remains a significant challenge in reproductive medicine, imposing substantial emotional, clinical, and financial burden on affected couples. RIF is generally defined as failure to achieve pregnancy after multiple transfers of good-quality embryos, yet progress in the field has been constrained by the absence of a universally accepted definition and by the inherent complexity of investigating the embryo-endometrial interface. Recent evidence has prompted a reappraisal of the factors contributing to RIF, distinguishing it from the mechanistically distinct entity of embryo-intrinsic failure (recurrent embryo arrest and poor embryo development). This narrative review synthesises epidemiological data, evaluates the evidence base for available diagnostic modalities, and appraises current management strategies, including the controversies surrounding adjunctive treatments. We propose a registry-corroborated, age-stratified definition of RIF - the ABCDE framework - and a five-tiered, clinically applicable management algorithm that integrates the available evidence with expert recommendation to guide clinicians within a shared decision-making framework. Future directions include prospective multicentre validation, incorporation of artificial intelligence for risk prediction, and development of novel therapeutic approaches targeting the embryo-endometrial interface.