What is the current evidence-based approach to recurrent implantation failure after euploid embryo transfer?

Before investigating, establish that failure was genuinely unexpected — see the definition question. Many patients labelled RIF have had too few good-quality embryos transferred for failure to be surprising. Where investigation is warranted, the areas with reasonable support: Uterine cavity. Hysteroscopy identifies and allows treatment of polyps, adhesions, submucosal fibroids and septa that ultrasound may miss. This has the clearest rationale of anything on the list. Chronic endometritis. Diagnosed on endometrial biopsy with CD138 immunohistochemistry. Antibiotic treatment where confirmed is reasonable, with evidence of moderate quality. Embryo factor. Review of laboratory performance, embryo quality trends, and consideration of PGT-A in selected cases — noting PGT-A does not improve the chance of a live birth for most patients and carries its own trade-offs. Male factor. Sperm DNA fragmentation in selected cases. Thyroid function and, where relevant, antiphospholipid antibodies. Where the evidence does not support routine use: endometrial receptivity testing (HFEA red-rated), immunological testing and immunotherapy, inherited thrombophilia screening, endometrial scratch, and intralipid or IVIG. The counselling challenge is that patients in this position are highly motivated to try anything. That makes it more important, not less, to be explicit about which interventions have evidence behind them.