they thaw one (or 2 in case you are transfering two) at a time and scientist watches to see if they are dividing.They told me it only takes an hour or so to know if the embryo is suitable. If one doesn't make it they take the next in line.

All grades I have heard can blossom into a miracle. I have heard of very poor embryos going the distance and the previous best not from same EPU !

Regarding the grading all fertility centres grade differently and some don't tell you. WFC just told us cells' no.s and that you can't 'really tell' in early stages. Like they can't tell or see DNA structure either.

Usually, the more cells the better - so at day 2 - approx 4 to 8 cells 4 is good to 8 cells is great - after this when cells divide more prolifically after day3 -sometimes slow embryos might only have 2 cells on day 2.
When they defrost - some cells can be lost so a 4 cell might go back to 3 cells ... then starts dividing again.

With blasts I am not so sure but they could see our embryos starting to hatch and in our case the scientist said the last 3 embryos wouldn't freeze well or there was a problem with structure ... not 100 % sure of the wording but part of it starts to divide into baby/sac etc and rest the pre-placenta for implantation. Implantation starts to happen day 6-7 or so.