Preparing and Timing of the Endometrium in Modified Natural Cycle Frozen-thawed Embryo Transfers

The increasing use of FET emphasizes the importance of preparing and timing the endometrium in FET cycles, however there is no consensus on luteal phase progesterone supplementation in mNC-FET and the optimal day of blastocyst warming and transfer. The aim of this multicenter RCT is to assess the effect of progesterone supplementation in hCG-triggered mNC-FET and the effect of embryo thawing and transfer at hCG+6 or hCG+7 days, respectively. In total 604 patients will be included with n=151 in each of the four study arms. The primary outcome is live birth rate per transfer (LBR) and the goal is to show a 10% increase in LBR after progesterone supplementation and to assess whether blastocyst warming+transfer 6 days after hCG trigger is superior to 7 days after hCG trigger in mNC-FET.