There has been some discussion on the DE Early 2014 thread and possibly others about this. I asked our current RE about it. He's agreeable to a subcutaneous injection of hCG during a FET cycle prior to or even at time of transfer, but when I asked about his views on the intrauterine injection at the time of transfer to assist with implantation in our case (we have some immune issues going on and I've got a history of 6 early pregnancy losses from immediately after implantation to 10 weeks since having a live birth in early 2012), his response was not to recommend it in my case. In case it's of assistance to others, I thought I'd post the response to this particular question for others' consideration.
Essentially, he said that "the hCG infusion has been used for women who fail to develop an adequate uterine lining" (he noted that this is not a problem in my case). He then added: "The [intra-uterine] injection route probably maintains circulating levels of hCG for a longer time as the endometrium actively absorbs and metabolizes the hCG hormone more quickly. I also have a concern about placing excess liquid into the endometrium as it may be a conduit for embryo movement."
This last part is important in my view - there is an art and science to embryo placement in the uterus. I've watched this RE place our last embryo (the cycle failed but it did implant - immune issues likely at play though one will never know for sure) and it was perfect for what's recommended vis-a-vis success rates. I find the research compelling about higher rates but the above explanation has satisfied me not to pursue it in my case if/when we do another FET. Hope this is of some insight or assistance to others or helps you have a meaningful conversation with your REs if this might be an option for you.
