Timing of frozen-thawed embryo transfer after controlled ovarian stimulation in a non-elective freeze-all policy

Controlled ovarian stimulation (COS) is a key determinant of in vitro fertilization/intracytoplasmic sperm injection (IVF/ICSI) success, in which multiple follicles develop simultaneously and plenty of oocytes are retrieved within one cycle. Unfortunately, the supraphysiologic hormonal concentrations during this process could also lead to reduced endometrial and subendometrial blood flow (1), advanced maturation of the endometrium (2), altered endometrial immune environment (3) as well as disrupted gene expression profile in relation to endometrial receptivity (4). These detrimental effects consequently translate into the less favorable outcomes of embryo implantation and pregnancy rates in fresh transfer cycles (5,6)