Hi Sarah,
There is another variable not mentioned on this thread so far, that can really affect the success rate of a transfer. That is whether or not the embryo is chromosomally normal. Statistically, even donors in their twenties can produce as many as 50% bad eggs. !! If these eggs fertilize, then their chromosome counts are not correct (monosomy or trisomy results), and either they won't implant, will implant with early miscarriage, implant with late miscarriage, etc.
If the embryo's have been tested for a panel of chromosomal abnormalities, via pgd (pre-implantation genetic diagnosis), then the only embryo's that will be transferred will be the ones which did not appear to be flawed (doomed).
The last clinic I used - the one where I got my twins, finally - had an excellent SART report. This was RSC in La Jolla, not the cheapest clinic but good. 84% success donor eggs with fresh cycles, ~80% with frozen. but SART does not discriminate between cycles with or without pgd filtering out the bad embryo's. At the top of my clinic's SART report, it mentioned 47% of cycles used pgd. That's important to note when deciding on a clinic based on success rates - did they use pgd a lot?
The pgd panel testing is expensive - from 4-7K I was cycling, depending on the clinic - and often not encouraged by RE's because of the expense and because donor eggs statistically have more normals anyway. If you don't mind risking putting back defective embryo's that's fine, you can just keep transferring, especially if your clinic gets mosts of the embryo's to make it to freezing. In the 3 DE cycles I did, my 23 yr old donor had 2 normals out of 11 embies, and my 27 yr old donor had 7 normals out of 11 and 5 normals out of 11. That's about a 42% average - not too far off the 50% normal average I'd heard for young women in their twenties. If I hadn't had any make it to freeze, and just blindly put some back at transfer each time... the odds for any embie taking would have been less than 50%. Testing them first changes the odds, to the extent that the clinic I used was concerned about putting back more than 2 pgd'd embies - their twin rate from putting back 2 was around 40%, so they worry about triplets from transferring 3...Instead, I put back 3 tested normals (one slow-growing, so they considered it less likely to implant so reluctantly let me put it in too), initially had 3 implant (!), and then two made it to the heartbeat and later live birth.
Good luck. IVF can be a long haul - even with donor eggs it can take several cycles - but your persistence plus a good clinic will improve your odds considerably.
AsliSuha