Comparative Embryo Development Outcomes following Extending Embryo Culture to Day 6: A Retrospective Cohort Study.

BackgroundPast studies have shown that culturing slow-growing embryos from day 5 to day 6 may increase vitrification yield. This study aims to evaluate if the proportion of embryos eligible for vitrification increases by growing embryos not vitrified by day 5 to day 6.Materials and methodsIn this retrospective cohort study, a Canadian tertiary-care clinic-based cohort was identified between August 2019 and December 2020. In vitro fertilization (IVF) cycles involving autologous oocytes with at least one viable day 5 embryo were selected for inclusion. We compared embryo developmental outcomes of IVF cycles performed before and after an embryo cryopreservation policy change. Prior to March 2020, good-quality day 5 blastocysts of any stage were eligible for vitrification, and after that date, good-quality expanded blastocysts on either day 5 or day 6 were eligible. The primary outcome is the comparative proportion of embryos eligible for vitrification. The secondary outcome is to identify embryo, maternal and cycle factors that are predictive of day 6 vitrification.ResultsA total of 3,438 viable embryos across 679 consecutive IVF cycles were included in this study. After the policy change, we found similar mean proportions of blastocysts eligible for cryopreservation (46.9% per IVF cycle in group 2 vs. 44.4% in group 1, mean difference 0.025, 95% confidence interval -0.021 to 0.071, P=0.28). The mean number of cryopreserved embryos were significantly higher in group 2 (mean 2.2 vs. 1.7 embryos, P=0.007). Factors that predicated an embryo's progression to day 6 included: younger age of egg provider, presence of an early blastocyst on day 5, and cycles involving surgically-retrieved sperm.ConclusionA cryopreservation policy change to include good-quality full and expanded day 6 blastocysts while avoiding to vitrify early blastocysts on day 5 yielded comparable proportions of embryos eligible for vitrification per IVF cycle.