As a different perspective - I had several transfers of day 5 or 6 blastocysts. Every single one of them ended in miscarriage. Out of 11 blastocysts, only four ever survived being frozen and thawed to be transferred.

My son is the result of transferring a fresh day three embryo. After my FS swore to us that our only hope of conceiving and staying pregnant was with a frozen blast as growing to blast and freezing and thawing would "weed out the ones that won't ever make it". We proved he was wrong. For some, blasts are better. For others, getting that embryo back into your uterus as soon as possible is better. You won't know what's better for you until you try it and find what eventually works (and sadly, it's not guarantee that it will work again next time).

The thing with averages and chances is very general - what works for the majority of women may not work for YOU. If what you've done in the past hasn't been working, it may be a good idea to try something different.

BW