Progesterone plays a key role in implantation through several mechanisms such as endometrial differentiation, myometrial quiescence or immune modulation. It has an essential function for the onset of pregnancy and is thus widely used in luteal support of ART cycles. Due to an increasing number of publications warning of obstetrical complications related to the absence of the corpus the artificial cycle in FET should probably be reserved for certain patients where no other alternatives are available for endometrial preparation. Future research will aim at defining properly the optimal progesterone threshold depending on the progesterone administration route, especially in case of multiple administration routes. There is still much to be explored for the individualization of luteal phase support in cycles with several corpora lutea. Some authors suggested closely monitoring the serum progesterone (‘luteal coasting’) so as to individualise the luteal phase support (87), but more trials are needed to precisely determine how to monitor serum luteal progesterone and when and how to adjust luteal phase support, differentiating cycles with hCG trigger and with GnRH agonist trigger.
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