The fate of frozen human embryos when transferred either on the day of thawing or after overnight culture

ObjectiveTo study the performance of thawed zygotes and cleavage stage embryos transferred either on the day of thaw or after overnight culture.MethodsA retrospective study of 864 frozen embryo transfer cycles. Cryosurvival rates per thawed embryo and implantation rates were analysed for embryos frozen on Day 1, Day 2 or Day 3 relative to oocyte collection (Day 0) and transferred on the day of thaw or after overnight culture, together with clinical pregnancy rates and prevalence of multiple gestations.ResultsSurvival of Day 3 embryos was significantly lower than those frozen on Day 1 (P=0.017) or Day 2 (P=0.015). Following overnight culture, resumption of mitosis of zygotes was more frequent than Day 2 (P=0.000) which are in turn higher than Day 3 (P=0.000) embryos. The implantation rate for Day 2 embryos dividing overnight was significantly higher than those that did not divide for women <35 yrs (P=0.001) but not those women <35 yrs (P=0.055). There were no differences in the implantation rates for those dividing or not after culture, for embryos frozen on Day 3 for women <35 yrs (P=0.254) or <35 yrs (P=0.403).ConclusionsLater cleavage stage post-thaw embryos survive and resume mitosis less frequently compared to earlier stages. Embryos not resuming mitosis after culture overnight can implant, particularly Day 3 embryos, suggesting that they can further increase the cumulative pregnancy rate per oocyte collection and that discarding them is wasteful. Overnight culture is best used for logistical reasons rather than a strategy to improve pregnancy rates.