Hello,
For us, this was a critical question (we had 5 failed transfers) - and all I really know is that there is a complex, and not fully understood chemical reaction between the lining and the embie...for us this was prevented by some nasty little antibodies (but again 'they' dont really know how this happens, or why the treatment provided works).

So for me it is just a little bit of magic that happens when your little one is ready to join you in this world. Wish we knew more as we could then reduce the journey we go through, but maybe somethings will just never be known!

Wishing you all the best with your upcoming FET!

FG