Progesterone (P4) is crucial for the achievement and maintenance of a pregnancy and with rising numbers of frozen embryo transfers (FETs) performed worldwide, the search for the 'optimal' P4 levels in HRT FET cycles became a focus of research. Certainly, measurement of systemic P4 levels is an easy applicable tool and P4 levels, considered as being too low, could be addressed by changing and/or increasing exogenously administered P4. However, the question must be raised whether the sole measurement of systemic P4 levels is reflective for the endometrial status and the endometrial receptivity in HRT FET cycles, since systemic P4 levels do not reflect the dynamic of the endometrial changes, deemed necessary to prepare the endometrium for implantation. Moreover, different types of P4 administration routes will exhibit distinct different patterns of P4 release, affecting the process of secretory transformation and last but not least, embryonic factors are almost fully neglected in this concept. This opinion article aims to raise critical points towards the 'sole' focus on systemic P4 levels in HRT FET cycles and raises the question whether 'serum P4 measurements are truly representative for the identification of an adequate luteal phase in HRT FETs'?.