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Frozen-thawed blastocysts retain their develpmental capacity much more than frozen-thawed cell stage embryos. The implantation rate for FET blastocyst runs around 20% per embryo, while FET cell-stage embryos are around 5%.
When freezing any cell (including embryonic cells), it is necessary to remove as much water from the cells as possible and replace the water with a cryoprotectant. Remember the old science expereiment where you fill the ice cube tray to the very top and put it in the freezer. When the water is freezes, the ice cubes come out over the top of the tray. This was used to demonstrate that water takes up more space as ice than when it was water. The same thing is true of the water inside the cells. If you don't remove as much water as you can, the cells blow up during the freezing process. Blastocyst thawing survival is significantly better than cell-stage embryos. The blastcoel cavity inside the blastocyst contains mostly water. This water must be removed as well as the water from the cells. Prior to freezing, the blastocst is "deflated" into a compact ball of cells, similar in appearance to an embryo at the compacted morula stage.
O.K. To answer your question. The way you can tell if a blastocyst stage embryo has survived the thaw is that it will still appear compact. If there was cryodamage, there will be a lot of debris (blown up cells) around the compacted embryo. A little debris is still O.K., but if more than 50% of the embryonic cells "blew up" during freezing, the chance of implanting is slim. The good news is that blastocyst embryos survive the freeze-thaw process very well. If they were good quality blastocysts prior to the freeze, there's a 90+ survival rate.
And everbody knows that girls survive better than boys. Just kidding. Good luck.
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