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 culture in 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.
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