Historically, embryo transfer was carried out 2 days (approximately 48–54 hours) after oocyte retrieval, but transfer has been carried out from as early as 1-hour post-ICSI to 5 days later, at the blastocyst stage. Trials of zygote transfer on Day 1 also achieved acceptable pregnancy rates. It seems that the specific timing of transfer may not be crucial for the human implantation process. On Day 2, cleaved embryos may contain from two to six blastomeres. Embryo transfer 1 day later, on Day 3, or on Day 5 at the blastocyst stage is advocated as a means of selecting embryos with higher implantation potential, by the elimination of those that arrest at earlier cleavage stages in vitro.
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