This was a retrospective cohort study conducted at the Department of Reproductive Medicine Center in the Third Affiliated Hospital of Guangzhou Medical University, Guangzhou, China. From April 2014 to December 2017, a total of 4404 patients were enrolled in this study. Inclusion criteria included: 1) women were 20 to 40 years of age, 2) had a normal menstrual cycle (defined as a spontaneous cycle length of ≥21 days and ≤ 35 days), 3) basal FSH<12mIU/ml, 4) the first FET cycle after whole embryo freezing using vitrification method, 5) COS regimens during IVF/ICSI cycle was the gonadotrophin releasing hormone agonist (GnRHa) or GnRH antagonist (GnRHant) protocol. Exclusion criteria included the following: 1) oocyte donation and cycles with preimplantation genetic testing (PGT), 2) patients were diagnosed with polycystic ovary syndrome (PCOS) or ovulation disorder, 3) known uterine anomalies confirmed by 3-dimensional sonography and/or hysteroscopy, including endometrial polyps, septal uterine cavity, submucucosal fibroid, etc.,4) the presence of hydrosalpinx not corrected surgically prior to FET, 5) uncontrolled endocrine and/or immune disorders or other systemic diseases, including hypertension, diabetes, thyroid disease, hyperprolactinemia, antiphospholipid syndrome, systemic lupus erythematosus, etc. Each patient has signed a informed consent on obtaining and analyzing their clinical data prior to the initiation of IVF/ICSI-ET treatment.
The FET cycles were divided in two groups according to the interval between withdrawal bleeding after ovum pick-up (OPU) and the start of the first FET. Immediate FET: women whose endometrial preparation of FET cycle performed within the first menstrual cycle following withdrawal bleeding; and delayed FET: women who had at least one menstrual cycle before initiation of endometrial preparation,and the transferred embryo were cryopreserved for less than 6 months. Furthermore, each group was divided into two subgroups according to COS protocols, and the reproductive and neonatal outcomes were analyzed between the immediate and delayed FET groups following the same COS protocol.
The FET cycles were divided in two groups according to the interval between withdrawal bleeding after ovum pick-up (OPU) and the start of the first FET. Immediate FET: women whose endometrial preparation of FET cycle performed within the first menstrual cycle following withdrawal bleeding; and delayed FET: women who had at least one menstrual cycle before initiation of endometrial preparation,and the transferred embryo were cryopreserved for less than 6 months. Furthermore, each group was divided into two subgroups according to COS protocols, and the reproductive and neonatal outcomes were analyzed between the immediate and delayed FET groups following the same COS protocol.
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