When similar live birth rates do not mean protocol equivalence in frozen embryo transfer.

Similar live birth rates in frozen embryo transfer (FET) trials can be over-read as evidence that natural, modified natural, and programmed endometrial preparation protocols are biologically equivalent. This interpretation is not warranted when cancellation, delayed transfer, and rescue switching are built into care. Under these conditions, randomization compares starting strategies that unfold through managed clinical pathways, not uninterrupted protocols completed exactly as assigned. Using recent FET trials as practical examples, this Clinical Opinion argues for a narrower and more constructive interpretation: similar live birth supports pathway effectiveness only for the estimand that was actually analyzed. Clinicians should ask whether the reported effect reflects assignment to an initial strategy, completion of the first assigned protocol, or a hypothetical no-switch contrast. Cancellation and rescue switching should be treated as outcomes; pathway burden should be reported alongside live birth, and safety outcomes should use denominators aligned with the stage at which harms occur. These FET-specific reporting recommendations apply existing CONSORT, CONSORT Harms, and ICH E9(R1) principles to trials in which cancellation or switching can materially affect interpretation.