Discussion

This study aimed to evaluate the success rates of two consecutive SETs in patients with a good prognosis compared with the transfer of two embryos in one frozen-thawed cycle, and demonstrated that the first approach is superior in terms of ongoing PRs. The strategy of transferring embryos one by one results in higher ongoing PRs with the advantage of avoiding multiple gestation. Although the use of a freeze-only strategy in patients at a risk for OHSS has been described (5, 25-27), studies on the freeze-only strategy have compared the method with fresh transfers, and no consensus has been reached thus far (8, 9, 28, 29).
The association of the freeze-only strategy with subsequent consecutive frozen-thawed SETs has not been extensively studied. A study evaluating the association between the freeze-only strategy and SET in women with hypogonadotropic hypogonadism showed that SET is an effective strategy for decreasing the incidence of multiple conceptions while maintaining satisfactory live birth rates (50.5%) (30). He and colleagues compared single and double frozen-thawed blastocyst transfers after a freeze-only strategy and found similar cumulative PRs (31). Our study found fewer multiple pregnancies with eSET, as expected; however, we also observed that two consecutive frozen-thawed SETs resulted in higher ongoing PRs than the transfer of two blastocysts in one frozen-thawed cycle. The regression model also confirmed an association of two consecutive SETs with a higher chance of implantation, which may indicate that transferring blastocysts one by one in a freeze-only strategy is better than transferring two blastocysts together.
A number of variables are associated with the embryo implantation potential, and they have been the focus of several studies. Not only the embryo quality evaluated using conventional morphology, time-lapse morphokinetics, or preimplantation genetic test (32) but also the endometrium status (33) and embryo-endometrial synchrony are crucial factors for a successful implantation (34). Studies from Simon and collaborators have investigated the progesterone action, endometrium gene, receptors, and protein expression based on data from the endometrial receptivity array (35-39). More recently, the endometrium microbiome and its association with embryo implantation have been studied by the same research group (40-42). Although the association of endometrium gene expression or microbiome with the implantation rates is controversial in the literature (43), it is clear that there is a variability in the endometrial condition between cycles according to the patient’s clinical condition or changes in the treatment approaches. These variations justify the greater chance of implantation when performing consecutive SETs compared with one DET. However, the large diversity of variables involved in embryo implantation, with respect to both the embryo and endometrium, makes the complete elucidation of that process impossible.
The vitrification technique is able to maintain the embryo implantation potential (44, 45), which, in turn, allows the performance of a freeze-only strategy and endometrium preparation for embryo transfer in natural or hormone replacement cycles. This was confirmed by an extensive study evaluating > 20,000 freeze-only cycles and FETs, in which all embryos transferred from that stimulation cycle were considered. The study demonstrated that the mean cumulative live birth rate was 50.0% and dependent on the number of oocytes collected and the patient’s age, suggesting the applicability of the freeze-only strategy for the general population (46). However, that study did not consider the number of embryos transferred, which allowed us to evaluate the association of the freeze-only strategy with SET. Also confirming our hypothesis of better clinical outcomes after a freeze-only strategy plus SET, the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology (ASRM/SART) recommended SET for patients with a good prognosis aged < 38 years, except in cases in which the patient had previously experienced several cycle failures and for whom DET was suggested (47).
Another study evaluated retrospective data about clinical PRs before and after the ASRM/SART guidelines, as their clinic policy is to transfer no more than the recommended number of embryos. The study assessed patients < 38 years old using their own eggs and without preimplantation genetic testing, before (mean of 1.3 embryos transferred per patient) and after (all embryos transfers were single) guideline revision. The outcomes showed that SET was very efficient in this population, as the overall live birth rates were maintained at around 50% after the reduction in the number of embryos transferred, and the twin PR decreased from 14.2% to 2.5% (48). This study also supported our hypothesis of the considerable advantages of transferring single embryos, highlighting the absence of reduction in PRs in SET and avoiding multiple pregnancies after a reduction in the number of embryos transferred.
Owing to the retrospective nature of our study, we adjusted our statistical analysis for confounders. However, we cannot exclude the possibility of residual interfering factors such as endometrium status or embryo ploidy, as these conditions were not evaluated in the cycles included in our study. A limitation of our study lies in the fact that not all patients who had failed eSET in the first transfer proceeded to a second SET cycle. Nevertheless, the calculation used in our study estimated the cumulative ongoing PRs as if all women who had a failed first SET had a second SET, confirming a clear advantage of transferring consecutive embryos one by one over DET. Another point to consider is that the decision about the number of embryos to be transferred and the embryo quality were not controlled. As a clinical routine, the number of embryos transferred was determined in a shared decision-making process between patients and doctors, after explaining the advantages and disadvantages of each situation. Thus, the choice of the number of embryos to be transferred could have been influenced by the quality of the blastocysts available and the couple’s preferences. Although our practice prioritizes transferring the best-quality frozen-thawed blastocysts available, data on the quality of embryos transferred were not available for the analysis. All cycles included were elective (with at least one surplus frozen blastocyst) and the demographic characteristics of our study groups were similar, which made the comparisons possible.