worrywart I am cetainly not up with the IVF statistics so can't help you there. I do know alot of people who have fallen pregnant with IVF who have had unexplained infertility. I think why the success rate isn't as high is because when you don't know what the problem is you can't try and fix it so you just have to have the embryo transfer. A certain percent of people know why (ie PCOS, Male sperm reason, progesterone problems, tube's damaged) so during the process they can try their best to accommodate the needs of the people based on what the problem is preventing pregnancy. Unfortunately when you don't know what is preventing pregnancy occuring their is nothing such to fix if that makes sense but it does not mean that IVF wont work for you. During the IVF process they might discover why you have not conceived and provide something else in addition to help you fall pregnant. I am not sure the statistics are for known fertility problems verse unknown infertility with successful IVF. I know unexplained infertility is lower but their are certainly many people where IVF has been successful with unexplained infertility.
I have been looking into different IVF clinics myself (yet to head down that track) The statistics on the web site of the IVF clinic I would use say almost 80% of their clients under the age of 40 have had a successful pregnancy resulting in a living child by their 3rd stimulated cycle. So I guess it doesn't give you any indication of how many FET have occured in that time. This info came of the IVF Sydney website so I don't know how accurate it is. I am yet to see the FS.