Impact of Previous-day versus Same-day Warming of Cleavage-stage Embryos on Reproductive Outcomes: A Retrospective Cohort Study.

Background: Studies have assessed the impact of culturing warmed cleavage-stage embryos to blastocyst in frozen embryo transfer (FET) cycles; there is a paucity of studies that have evaluated the impact of warming of day 3 embryos and culturing for a day before embryo transfer. Aim: The purpose of this research was to study whether warming of day 3 embryos and subsequent transfer on day 4 versus same-day warming and transfer of day 3 embryos improves the treatment outcomes following a FET. Settings and Design: We conducted a retrospective cohort study in the Department of Reproductive Medicine and Surgery at Christian Medical College, Vellore, India. All couples who underwent FET at our centre between January 2016 and December 2023 were screened for eligibility. Materials and Methods: The patients were divided into two groups according to the stage of the embryo that was transferred: warmed and transferred the same day as a day 3 embryo. The other group in which embryos were warmed the previous day of embryo transfer, cultured for 1 day and were transferred as day 4 embryo. The study groups were matched for age and body mass index by case-control matching. After case-control matching, 209 FET cycles were analysed in either group. The primary outcome was live birth rate (LBR). Statistical Analysis Used: A parametric t-test or non-parametric Mann-Whitney test was used to find the difference between the groups. The Chi-square and Fisher's exact tests were used to find the association between categorical variables. Univariate and multivariable analysis was performed with a multiple logistic regression model to assess the potential impact of those parameters on LBR. Results: The clinical pregnancy rate per transfer (41.1% vs. 38.2%, P = 0.548) and LBR per transfer (29.1% vs. 27.7%, P = 0.745) were comparable between day 3 versus day 4 transfer groups. Although the cycle cancellation rate was comparable in both the groups (0.8% in day 3 vs. 1.4% in day 4, P = 0.673), cryosurvival of embryos was higher in the day 3 group versus the day 4 group (86.3% vs. 81.7%), and the difference was significant (P < 0.001). Multivariate logistic regression analysis demonstrated that the likelihood of achieving live birth did not depend on day of embryo transfer (odds ratio: 1.059, 95% confidence interval: 0.688-1.628, P = 0.793). Conclusion: Warming cleavage-stage embryos and culturing for 1 extra day did not show any significant difference in FET outcomes.