Natural cycle frozen-thawed embryo transfer-can we improve cycle outcome?

Several replacement protocols for frozen-thawed ET (FET) exist, with no advantage of one protocol over the others. In the present study, we aim to evaluate the outcome of natural cycle FET with modified luteal support. All consecutive patients undergoing natural or artificial hormone replacement (AHR) day-2/3 FET cycles between May 2012 and June 2015 in our IVF unit were evaluated. While AHR FET cycles were consistent, those undergoing natural cycle FET received progesterone luteal support, and from June 2014, patients received two additional injections, one of recombinant hCG and the other of GnRH-agonist, on day of transfer and 4 days later, respectively (modified luteal support). Patients' clinical characteristics and laboratory/embryological variables were comparable between those undergoing natural vs. AHR cycles, during the earlier as compared to the later period. Moreover, while implantation, clinical, and ongoing pregnancy rates were significantly higher during the later period in patients undergoing the natural cycle FET with the modified luteal support (31, 51, and 46 %, respectively), as compared to natural (17, 26, and 20 %, respectively), or AHR FET in the late study period (15, 22, and 17 %, respectively), the natural cycle FET without the additional two injections yielded the same results, as the AHR cycles. We therefore suggest that in ovulatory patients undergoing FET, natural cycle FET with the modified luteal support should be the preparation protocol of choice. Further large prospective studies are needed to elucidate the aforementioned recommendation prior to its routine implementation.