Frozen Embryo Transfer Success – Important New Information!
While many studies have looked at frozen embryo transfer success rates, few studies have stratified frozen embryo success rates based upon the outcome (successful pregnancy or not) of the fresh embryo transfer from the same group of embryos. A recent study published in Reproductive BioMedicine Online - Feb. 2014, showed a significant difference in clinical pregnancy rates in the frozen embryo transfer cycle based upon whether the fresh cycle resulted in pregnancy.
In their study the overall fresh embryo transfer clinical pregnancy rate was 44% and the frozen clinical pregnancy rate was 34%. The study found that when the fresh embryo transfer was successful, the frozen embryo transfer with embryos from the same cohort was significantly less likely to result in pregnancy. In the patients who failed to conceive in the fresh embryo transfer cycle, subsequent transfer of frozen embryos from the same cohort achieved a clinical pregnancy rate of 43%. The conclusion reached was that when fresh cycles are unsuccessful, the remaining frozen embryos from the same cohort have the same prospects for achieving a clinical pregnancy as the fresh cycle. Frozen cycles following successful fresh embryo transfer cycles had significantly lower clinical pregnancy rates (22%) compared with the fresh embryo transfer (43%).
When an in-vitro fertilization cycle has gone well enough to generate fresh embryos for transfer as well as frozen embryos, at least two separate chances to achieve a clinical pregnancy have been achieved. This study suggests that the selection of the embryo(s) for transfer is imperfect. Although the selection of the embryo for transfer is based on characteristics of early embryonic growth as well as the microscopic appearance of the embryo, it is a process that is less than precise. In cases where the best embryo was inadvertently not selected, pregnancy fails to occur. In these situations, it is probable that the “best” embryo was frozen, which would explain why the subsequent frozen embryo transfer results in a clinical success rate that is comparable to the fresh embryo transfer success rate for a given program.
The take home message from this study is that there is a difference in the expectation for success with frozen embryo transfer based on whether the fresh embryo transfer from the same cohort of embryos was or was not successful. When transferring frozen embryos in patients who have previously achieved success with fresh embryos from the same cohort, it is reasonable to temper expectations to some extent based on the data presented in this study. On the other hand, when counseling patients who have failed to achieve a successful pregnancy in the fresh embryo transfer cycle, they may be advised and encouraged that the expectation for success is equivalent to what would be expected in a fresh embryo transfer. Other programs may be encouraged to review their own frozen embryo transfer data to look for similar