Role of serum progesterone levels in frozen embryo transfer: an update

PURPOSE OF REVIEW: Progesterone plays a pivotal role in implantation and ongoing pregnancy. In the context of assisted reproductive technology (ART), and in particular frozen embryo transfer (FET), increasing evidence suggests that serum progesterone levels strongly influence reproductive outcomes. This review summarizes the latest findings regarding the optimal range of serum progesterone concentrations and their implications across different endometrial preparation protocols. RECENT FINDINGS: Observational and interventional studies have consistently shown that suboptimal mid-luteal serum progesterone levels are associated with lower implantation and live birth rates in FET cycles. The need for exogenous supplementation varies according to the type of endometrial preparation, with hormone replacement therapy cycles being particularly sensitive to inadequate progesterone exposure. Emerging data support the concept of individualized luteal support, including serum-guided adjustments or alternative routes of administration in cases of suboptimal serum progesterone levels. SUMMARY: Adequate serum progesterone concentrations are critical for optimizing outcomes after FET. Evidence supports a paradigm shift from empirical supplementation to precision medicine approaches based on serum thresholds and patient characteristics. Future research should aim to define universally accepted cutoff values, clarify the role of endometrial versus systemic progesterone, and refine strategies for tailoring luteal phase support in ART.