The Effectiveness of the Letrozole-induced Endometrial Preparation Protocol in Frozen-thawed Embryo Transfer (FET)

The goal of this single center, non-blinded, randomized controlled clinical trial to comparison of pregnancy outcomes and perinatal outcomes in patients undergoing IVF treatment (including ICSI) with FET with letrozole-induce endothelial preparation protocol versus natural cycles, hormone replacement protocol. The main questions it aims to answer are: * To investigate whether letrozole-induce endothelial preparation is effective in improving the live birth rate and clinical pregnancy rate. * To explore its possible impact on clinically important indicators such as spontaneous abortion rate, implantation cycle cancellation rate, days of endothelial preparation, and number of visits to the clinic. The study subjects were randomized into groups starting at D1-D3 of the menstrual cycle. The study subjects were stratified according to whether their menstrual cycles were regular or not, and were divided into the following endothelial preparation regimens according to the pre-prepared stratified zoned randomized group numbers: (1) regular menstrual cycles (25-35 d): letrozole ovulation-promoting cycles, natural cycles, and hormone-replacement cycles; and (2) irregular menstrual cycles (\<25 d or \>35 d): letrozole ovulation-promoting cycles, hormone-replacement cycle.