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

Past 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. In 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. A 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. A 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.