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Firstly, 9-10 cell embryos are not necessarily "good" embryos. Embryos must reach the blastocyst stage before they attain the ability to attach to the endometrium and implant. Embryos reach the blastocyst stage on Day 5-6 of development. Many of the embryos arrest development prior to reaching the blastocyst stage due to genetic abnormalities. Although embryonic arrest is more common prior to reaching the 8-cell stage of development, embryos can a do arrest development (usually a failure to undergo compaction) on Day 4 or Day 5 of development. The truth is that we know little about the developmental potential of embryos on Day 3 of development.
Attachment of the embryos to the endometrium occurs around Day 5-6. Implantation is a process that takes about 10 days to complete after the intial attachement.
In addition, there are many endometrial factors that can interfer with the implantation of "good" embryos such as adenomyosis (endometriosis of the uterine muscle), auto-immune disease and abnormal clotting factors.
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