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

Document Type : Original Article

Authors

1 Ottawa Fertility Centre/Division of Reproductive Endocrinology and Infertility, Department of Obstetrics and Gynecology, University of Ottawa, Ottawa, Canada. Ottawa Hospital Research Institute, Ottawa, Canada

2 Faculty of Medicine, University of Ottawa, Ottawa, Canada

3 Ottawa Fertility Centre, Ottawa, Canada

4 Ottawa Fertility Centre/Division of Reproductive Endocrinology and Infertility, Department of Obstetrics and Gynecology, University of Ottawa, Ottawa, Canada

Abstract

Background: Past studies have shown that culturing slow-growing day 5 embryos to day 6 may increase vitrification yield. This study aims to evaluate if growing embryos not vitrified by day 5 to day 6 increases the proportion of embryos eligible for vitrification.

Methods: A Canadian tertiary-care clinic-based cohort was identified retrospectively 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 full and 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.

Results: 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.

Conclusion: 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.

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