Clinical Outcomes of Transfer Vitrified-Thawed Day 4, 8-Cell Embryos into Endometrium Prepared for Day 3 Embryos

Objective: To determine the clinical outcomes of transfer of day 3, 8-cell embryos into endometrium prepared for day 3 embryos.Methods: This study performed a retrospective analysis of 1,190 retarded embryos. These embryos underwent extended culturein vitro to select and vitrify day 4, 8-cell embryos, followed by 176 frozen embryo transfer cycles (study group), matching 660 transfer cycles with single frozen day 3, 8-cell embryos as the control (control group).Results: The study group achieved successful implantation rates, clinical pregnancy rate, and live-birth rates (20.45%, 15.91%, 14.20%, respectively), although these were lower than that in the control group (30.45%, 27.42%, and 20.91%;P = 0.009, 0.002, and 0.046, respectively). The miscarriage rate was similar to that in the control group (4.55% vs. 3.33%,P > 0.05), and the mean birth weight of the study group was higher than that in the control group (3,556 ± 381 g vs. 3,311 ± 570 g,P = 0.012).Conclusions: Transfer of frozen day 4, 8-cell embryos into endometrial prepared for day 3 embryos can be a new and safe alternative for patients with delayed embryos.