For a pregnancy to occur, an euploid embryo at blastocyst developmental stage, a receptive endometrium and the synchrony of both is crucial. Many studies lately investigated the influence of the endometrial thickness and pattern on the artificial reproductive technology (ART) outcome, however, with conflicting results.

Further on, the measurement of the endometrial thickness was mostly performed either on the day of final oocyte maturation in stimulated cycles with fresh embryo transfer or on the day of progesterone administration in FET cycles. Progesterone is essential for the secretory transformation and compaction of the endometrium, prior to implantation. A recently published paper (Haas et al., 2019) however, evaluated the degree of endometrial compaction under the influence of progesterone in FET cycles and described, that a lack of certain endometrial compaction has a negative impact on the ongoing pregnancy rate. As in this study embryos of unknown ploidy status were transferred, the role of embryo ploidy on the outcome may bias the study results. In the herein presented study protocol we aim to investigate the influence of endometrial compaction in FET cycles in which euploid embryos are transferred. HYPOTHESIS: Lack of endometrial compaction after the start of progesterone leads to an impaired reproductive outcome.