Clinical Appraisal of "Micronized Vaginal Progesterone Dose and Serum Progesterone Thresholds Determine Reproductive Outcomes in Frozen-Thawed Embryo Transfer With Hormone Replacement Therapy"

This paper critically evaluates Sekiguchi et al.'s study of vaginal progesterone dose, serum progesterone thresholds, and reproductive outcomes in hormone replacement therapy frozen embryo transfer (HRT-FET) cycles. The study conflates descriptive, predictive, and causal aims without a clearly prespecified primary question, limiting interpretability. Major concerns include non-random treatment allocation, potential residual confounding, absence of a causal framework for covariate selection, unaddressed temporal confounding related to Japan's 2022 insurance reform, and exclusion of an available progesterone formulation. Additionally, formulation specific pharmacokinetics, single-time-point progesterone measurement, and internally derived thresholding limit comparability, predictive validity, and clinical applicability.