Sotto
Just before they do the embryo transfer they grade the embryos. There are at least 3 grades.

Grade 1's are the best and for memory a grade 1 embie has 8 well defined cells after 24hours (or it could be 48?) Not sure exactly. A grade 2 might have 6 cells, 7 cells or it might have 8 cells but also have some other fragments in it which means that it's ok but not as high a quality as a grade 1.

We've also had embies that were really poor quality - so poor that we didn't get a grade. They usually don't make it through the 48 hr mark.

Last time I had 10 eggs, and we got 4 embies, 2 between grade 1 and 2 and 2 between grade 2 and 3. We elected to put the two better ones back and freeze the other two. When we got back the next day all of our embies had flourished and we were upto 4 grade 1's - I have no idea how it works but the embryologists all seem to do this grading thing. I think the theory is that grade 1 embies do better in utero than lower grades. But it all seems to be a moveable feast.

Maybe other LTACers know more?

Good luck with your trigger. It's a bit nerve wracking getting the ampoules open so make sure your DH is with you to help calm you down.
Enjoy the bloom it gives your skin! You'll have a mini pregnancy glow for a few days. SEnding you baby dust

Editted to add. I just remembered what DH and I agreed on our first IVF attempt. THis might help you with your decision making about one versus two.

We decided that if we had a grade 1 we'd put 1 back, and if we had grade 2's we'd put 2 back. That kind of helped us on the day.