The effect of fresh IVF cycle characteristics on frozen embryo transfer (FET) outcomes

Objective: To determine the effect of fresh IVF/ICSI cycles on FET cycle embryo and pregnancy outcomes. Methods: This retrospective cohort study included data from the medical records of 104 FET cycles performed from January 2014 to December 2016. Embryos were previously vitrified and then thawed for embryo transfer. Statistical significance was established at p<0.05. The main endpoints were FET cycle survival and pregnancy rates. Results: A total of 104 FET cycles were analyzed for survival; 94 showed good progression and 84 achieved embryo transfers. Patients with secondary infertility achieved significantly higher pregnancy rates - 6/38 (15.8%) vs. 18/46 (39.1%) (p<0.018). Stimulation with FSH/LH resulted in more significant embryo progression, 38/48 (79.2%) vs. 28/46 (60.9%) in the FSH group (p=0.01). Patients who got pregnant from fresh cycles had the highest pregnancy rates in FET cycles (p<0.0001). Lower numbers of frozen embryos correlated with higher pregnancy rates (p=0.048). Embryos frozen on day 2 or 3 had the most significant progression (p<0.0001). Freeze-thaw intervals >12 months yielded higher pregnancy rates, 13/30 (43%), vs. 11/54 (20.4%) (p=0.025). Conclusion: Patient pregnancy in fresh cycles is a good prognostic factor for FET cycle success. Delaying FET by more than 12 months might result in higher pregnancy rates.