Luteal phase: New ideas on an old concept

In the last decades we have experienced, a worldwide steep increment on the demand of assisted reproductive treatments (ART). Therefore, the necessity to improve the performance of ART has been crucial to increase the pregnancy outcomes. Although research has pronominally focused on identifying the proper ovarian stimulation improving condition in IVF laboratories, luteal phase has possibly been the field in which research has been unproportionally deficient. Despite the strong evidence on the importance of the luteal phase support in IVF fresh and frozen cycles, there is still a lack of homogeneity on the best route of progesterone (P4) administration, the dose, and the surveillance of the luteal phase, suggesting that future research may be needed in this field. Independently on the ovarian stimulation (OS) or the frozen embryo transfer (FET) techniques, mostly vaginal and parenteral P4 are prescribed nowadays. However, in recent years the oral and subcutaneous administration of P4 were introduced and demonstrated to be safe and as efficient as the other routes. At this time point, the decision on which P4 route and dose should be used, in a tailored patient-approach, is still controversial, especially considering the safety and the tolerability of the P4 treatment. The aim of this narrative review is to provide evidence on the latest innovation on the luteal phase surveillance and management and to describe the different treatment options for the luteal support, not only in fresh, but also in frozen embryo transfer cycles which demonstrate an exponential increase the last decade.