What is the optimal timing of thawing for transferring vitrified-warmed cleavage stage of slow-growing embryos? A cohort retrospective study

Background: To evaluate optimal thawing time, the early thawing or the routine thawing 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 slow-growing embryos FET cycles in which the patients were aged < 40. Thawing cleavage stage slow-growing Day 3 embryos on either the 2nd or 3rd day after ovulation in natural cycle or the equivalent timing of the artificial cycles. Results For slow growing embryos, the clinical pregnancy rate of early thawing group (152/468 (32.5%)) was significantly higher than that of routine thawing 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 thawing group (39/170 (22.9%)) versus in routine thawing 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 thawing strategy as compared to the routine thawing, which suggested that the improvement of endometrium-embryo synchronism may correct the time difference brought by the slow-growing embryos.