Progesterone supplementation in the frozen embryo transfer cycle

Currently, different modalities with regard to endometrial preparation for frozen-thawed embryo transfer (FTET) are used, the natural and artificial cycles being the most common approaches. This review is aimed to update the current knowledge about progesterone supplementation in both types of protocols.Natural cycle-frozen-thawed embryo transfer (NC-FTET) is the favored option for women with normal ovulatory menstrual cycles and may be programmed following two different protocols: the 'true NC-FTET', associated with daily blood or urine luteinizing hormone measurements, and 'modified NC-FTET', triggering ovulation with human chorionic gonadotropin. Both methods of endometrial preparation show comparable reproductive outcomes. In artificial cycle-frozen-thawed embryo transfer, estrogen and progesterone are sequentially administered, being the option of choice for women with irregular menstrual cycles. Nowadays, no differences between the different formulations of progesterone have been observed. Furthermore, there seems to be no agreement on doses and duration of progesterone supplementation during FTET.We conclude that, according to the current available data, there is no superiority of any one regimen over another with regard to reproductive outcomes. Therefore, the final decision must be based on individualization of the treatment while considering patient characteristics prior to FTET, convenience, optimization of clinical outcomes and cost efficiency of the procedure.