ABSTRACT Globally, there is a paradigm shift towards frozen embryo transfers (FET). The protocols are divided into two categories based on the presence or absence of ovulation: natural cycle (NC) and artificial cycle (AC). Although pregnancy rates appear comparable in most studies, there is emerging evidence suggesting a potential association between AC-FET cycles and adverse obstetrical and perinatal outcomes. Suboptimal endometrial preparation and/or absence of the corpus luteum (CL) has been implicated as a potential aetiology. A shift towards NC-FET protocols has been explicitly propagated in the literature for safer maternal and neonatal outcomes. To optimise the NC-FET cycles with both true NC-FET and modified NC-FET, the role of progesterone as luteal phase support (LPS) is emphasised, though there is no consensus in the literature on using progesterone as LPS in these FET cycles. We provide a review based on the biological rationale and clinical evidence for progesterone as LPS in true NC-FET cycles and modified NC-FET cycles: whether it is required, can serum progesterone levels help in individualising LPS, and is there an optimum point in the luteal phase to begin progesterone supplementation?
Varma et al. (Mon,) studied this question.