What is optimal timing of warming for transferring vitrified cleavage stage of day 3 slow-growing embryos? A cohort retrospective study.

PURPOSE: To evaluate optimal warming time, the early warming or the routine warming time, for transferring vitrified-warmed and cultured overnight cleavage stage of the slow-growing embryos on day 3 in frozen embryo transfer (FET) cycle. METHODS: This was a retrospective cohort study from January 2017 to July 2018. A total of 705 FET patients aged < 40 years were included and 1486 embryos were formed, of which 1366 embryos were eventually transferred. RESULTS: For slow-growing embryos, the clinical pregnancy rate of early warming group [152/468 (32.5%)] was significantly higher than that of routine warming group (55/235 (23.4%)) [OR 1.39 (CI 1.06-1.81), p = 0.01], while there was no statistically significant difference in pregnancy loss in early warming group [39/170 (22.9%)] versus in routine warming group [16/62 (25.8%)] [OR 0.89 (CI 0.53-1.47), p = 0.65]. CONCLUSION: For slow-growing embryos, higher pregnancy outcomes were shown in early warming strategy as compared to the routine warming, which suggested that the improvement of endometrium-embryo synchronism may correct the time difference brought by the slow-growing embryos.