Defining recurrent implantation failure: a profusion of confusion or simply an illusion?

Recurrent implantation failure (RIF) is a poorly defined clinical scenario marked by failure to achieve pregnancy after multiple embryo transfers. The causes and definitions of implantation failure are heterogeneous, posing limitations on study design as well as the interpretation and application of findings. Recent studies suggest a novel, personalized approach to defining RIF. Here, we review the implantation physiology and definitions of the implantation rate, failure, and RIF.