This study, investigating whether cleavage transfers provide a benefit by avoiding transfer cancellations during extended culture and the relative efficacy of blastocyst and cleavage transfers in relation to the available number of zygotes, found that blastocyst transfers were not inferior to cleavage-stage embryo transfers among patients with few zygotes. Our findings are supported by Haas et al. [12] as comparable cumulative pregnancy rate per patient for patients with one or two cleavage-stage embryos was reported, regardless of embryo quality; however, when the pregnancy rates were analyzed per embryo transfer, blastocyst transfers resulted in higher pregnancy rates. Levi-Setti et al. [13] found that the cycle outcomes of patients (female age < 39, ≥4 zygotes) were comparable between blastocyst and cleavage transfers. Yang et al. [14], also demonstrated that the implementation of a time-lapse algorithm for cleavage transfers remains inferior to blastocyst transfers. Their findings were attributed to either embryo self-selection during extended culture [15] or better synchronization of the embryo and uterus [16]. In contrast, clinical outcomes of the blastocyst and cleavage transfers for patients with only one viable embryo on day 3 favored cleavage transfers [17], but the poorer outcomes from blastocyst transfers did not appear related to embryo loss in extended culture.
Most recently, De Croo et al. [18] retrospectively analyzed the live birth rates of four different embryo transfer strategies: cleavage transfers for all, blastocyst transfer for patients with > 9 zygotes, for patients with > 4 zygotes, and blastocyst transfer for all per oocyte collection cycle. They found that blastocyst transfers resulted in comparable live birth rates per retrieval among all the groups; however, the clinical outcomes regarding the number of zygotes available were not reported. In the present study, the effect of the zygote cohort size on the efficacy of embryo self-selection in extended culture was investigated. The clinical pregnancy rates for the patients with ≥ 6 zygotes after extended culture to the blastocyst stage were higher than those at the cleavage stage. One possible explanation for the poor cleavage transfer outcomes is that the conventional morphological criteria for cleavage embryos might be insufficient for choosing the most competent embryos [19]. In addition to the conventional morphological evaluation, the time-lapse selection of cleavage embryos resulted in lower implantation rates than in blastocyst transfers for patients with > 10 mature oocytes [14]. This result is understandable, given that more potentially competent embryos will be produced when the ovarian reserve and response are increased, thereby increasing the effectiveness of embryo self-selection. Recent studies support this conclusion, as clinical pregnancy and live birth rates have been shown to increase according to the number of eggs retrieved [20] and oocytes fertilized [21]. Consequently, the probability of finding at least one euploid embryo increases with the number of oocytes retrieved [22] and embryos biopsied [23]. The blastocyst cohort size is also associated with at least one euploid embryo being found [24]. Here, the improved implantation and pregnancy rates of blastocyst transfers for patients with a larger zygote cohort could be attributed to the production of more embryos with implantation potential and improved embryo selection properties resulting from extended culture.
The live birth [25] and clinical pregnancy [26] rates of first fresh embryo transfers are higher for blastocyst transfers, consistent with our study’s findings. However, for patients with a poor prognosis who produce a limited number of embryos, the current literature is insufficient to assist practitioners in developing embryo transfer date strategies. Despite the benefits of blastocyst transfers, it is speculated that an in vitro environment is inferior to an in vivo environment. Higher transfer cancellation rates of blastocyst transfers are mainly attributed to suboptimal extended in vitro culture conditions [27]. Per the UK National Institute of Health guidelines (NICE Guidelines), cleavage transfers are recommended to avoid transfer cancellations when few embryos are available [28]. The American Society for Reproductive Medicine also recommends avoiding transfer cancellations [10]. Performing a cleavage transfer eliminates the risk of transfer cancellation for those patients. To date, it remains unclear whether these patients benefit from cleavage transfers.
A major challenge of assisted reproduction is the treatment’s cost-effectiveness. Reducing the number of failed embryo transfers, thereby decreasing the time to conception, is critical [29]. To date, clinical pregnancy and live birth rates for blastocyst transfers are higher than those of cleavage transfers for first fresh embryo transfers [30]. Further studies have shown that blastocyst transfers reduce the mean number of cycles and days needed per live birth compared to those of cleavage transfers [8]. According to current knowledge, most aneuploid embryos show arrested development during extended culture [15, 16]. Aneuploidy rates of slow-developing embryos in extended culture are significantly elevated, regardless of patient age [31]. The clinical pregnancy and live birth rates of embryos with delayed blastulation and poor expansion patterns are also lower than those of fully expanded blastocysts [32]. Therefore, performing an extended culture for all patients may decrease the transfer of incompetent embryos, which will reduce costs and shorten the time to conception [33].
To our knowledge, the following methods used here were a novel approach to the dilemma of cleavage or blastocyst transfers for patients with few embryos. First, transfer date decisions were made independent of patient characteristics, and patients with a low zygote number were also allowed to extend the culture to D5. Second, transfer cancellations in the D5 group were not excluded from the analysis, they were counted as transferred but not pregnant to compare pregnancy rates per patient rather than per embryo transfer.
Blastocyst transfers are associated with a sex imbalance, favoring men and monozygotic twinning [34–36]. They are also associated with increased birth weights [37]. However, they are as safe as cleavage transfers in terms of pregnancy complications, obstetric outcomes, and congenital abnormalities [25, 27, 36, 38]. Consequently, patients should be thoroughly counseled about the potential benefits and risks of blastocyst transfers.
This study has several limitations. Although the embryo transfers were planned independently of patient characteristics, the study’s retrospective nature introduces potential bias, as the patient characteristics were not completely matched between groups. The patients receiving blastocyst transfers were younger with more mature oocytes. Furthermore, the ongoing pregnancy and live birth rates were not available for analysis. Further prospective studies are needed to verify these results.
In conclusion, blastocyst transfers provide significantly better results for patients with many zygotes, suggesting that it is preferable to perform extended culture and delay embryo transfer to find or select competent embryos. Moreover, extended culture seems to have no or negligible influence on embryo viability. Patients with few zygotes undergoing either D3 or D5 transfer have similar clinical outcomes. This can guide practitioners and patients in avoiding cleavage transfers that will not be successful.