It is interesting and even amusing to see how certain concepts come full-circle and even flip-flop, over time. The debate over fresh vs. frozen transfers is one of these.
For the first 20 years of IVF history, it was a general given that fresh embryo transfers were superior to frozen ones. Early attempts at freezing all the embryos and subsequently transferring them did not go well. Two theories are used to support this approach. The first is that freezing embryos would somehow sort out the better ones, as the less viable embryos would like not survive the thaw. A sort of embryo “stress test.” The second is that the uterine environment during a stimulated cycle is less conducive to implantation, so delaying the transfer may improve implantation.
Over the past few years, the belief in the superiority of frozen transfers has gained new interest. A few papers have been published showing improved implantation rates with frozen embryo transfers. Yet, not all clinics agree that this should be done on everyone.
There are some known variables that affect success rates with fresh and frozen embryos. First of all, it is clear that embryo survival after freezing/thawing is related to the intrinsic quality of the embryo. We see a very high survival rate and pregnancy rate with PGD normal embryos. Secondly, it has been shown that during fresh cycles, subtle rises in progesterone levels prior to the administration of hcg, severely affect implantation rates. This factor may not be addressed in some of the previously mentioned studies.
Given the above conflicting pieces of evidence, it is fair to say that frozen embryo transfers are probably comparable to fresh. Frozen transfers should not be viewed as less successful anymore. I use the Endometrial Receptor Assay (ERA) to see if a frozen transfer is likely to be better. Although not intended to be used this way, it seems to a helpful guide in individual patient management. If freezing all the embryos is recommended, the ERA should be strongly considered to help improve the outcome of the subsequent frozen transfers.