Impact of day 5 vs. day 6 blastocyst transfer on the pregnancy outcome of frozen-thawed donor recipient cycles

INTRODUCTION

Embryo cryopreservation have become routine procedures in human-assisted reproductive technologies and have managed to change the landscape in fertility treatment. By 2017, many centers have adopted embryo cryopreservation as their primary IVF therapy and perform few or no fresh embryo transfers. The two important reasons for cryopreservation are to achieve better endometrial receptivity when embryos are transferred in cycles without exposure to ovarian stimulation, and the ability to store the embryos while awaiting the results of preimplantation genetic testing. The practice of cryopreservation for frozen-thawed embryo transfer cycles (FET) is more appropriate for ovarian hyperstimulation syndrome prevention, medical issues, efficacy, or the desirability.^[1] Outcome from the several studies that compared fresh and frozen-thawed FET showed significantly higher clinical pregnancy rate with no increase in birth defects or development abnormalities.^[2]

However, a substantial increase in rate of multiple births is evident with the development of IVF techniques. To reduce the risk of multiple pregnancy, single embryo transfer (SET) and blastocyst culture have become a recent trend and are associated with significantly higher implantation rates, compared to earlier transfers of cleavage-stage embryos.^[3,4] Increased implantation rates maintain good pregnancy rates while transferring fewer embryos, thus reducing the risk of multiple pregnancy.

Identification of characteristics of blastocysts is important to refine the procedure of embryo transfer. Excellent pregnancy rates have been reported with fresh single blastocyst transfers.^[3,5] However, some embryos reach blastulation by day 5 and others not until day 6 or even day 7.

There have been various reports in literature about clinical outcomes associated with day 5 and day 6 blastocyst transfers. Results from a meta-analysis study indicated more favorable outcomes for day 5 versus slower developing day 6 vitrified blastocysts.^[6] However, when embryos of the same stage are compared, there would appear no significant differences.^[6] Contrary to these, few findings showed improved clinical results for day 5 versus day 6 blastocysts at the same developmental stage.^[7,8,9]

The prime objective of current study was to compare the pregnancy outcome for SET of day 5 versus day 6 frozen-thawed donor recipient embryo transfer cycles.

MATERIALS AND METHODS

RESULTS

In total, 414 FET were taken and all the embryos were non-preimplantation genetic tested (PGT) embryos. Out of which, 335 in the day 5 (group A) and 79 in the day 6 (group B) were analyzed. [TABLE 1] summarizes the comparison of statistics of group A and group B. Both the groups were comparable regarding average age of oocyte donor (25.03 vs. 24.97 years, p = 0.290), endometrial thickness (8.06 vs. 8.23 mm, p = 0.5164). More good-quality embryos were transferred in the group A (56.74% vs. 39.24%, p = 0.0033), whereas fair-quality embryos were more in group B (25.67% vs. 46.88%, p = 0.0001), but there was no difference between clinical pregnancy rate (50.74% vs. 49.36%, p = 0.825), ongoing pregnancy, and implantation rate (50.1% vs. 46.8%, p = 0.6357). However, we observed a significant change in pregnancy loss rate (17.05 % vs. 33.33%, p = 0.022), as shown in [FIGURE 1].

TABLE 1 Clinical Outcomes of D5 and D6 Frozen-Thawed Blastocyst Transfers

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FIGURE 1

% pregnancy loss in Group A (Day 5) vs. Group B (Day 6).

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DISCUSSION

This study examined whether the use of day 5 or day 6 thawed blastocysts could lead to different treatment outcomes. The data were analyzed retrospectively from a large sample size with thawed blastocyst transfer cycles, in which only high-quality blastocysts were frozen. Blastocyst freezing enhances the efficiency of IVF development by augmenting the chance of pregnancy per oocyte retrieval.^[10] Therefore, an understanding of the characteristics of blastocyst development could guide clinical practice to further improve IVF outcomes.^[11]

Different studies have shown that in fresh cycle blastocyst transfer on day 5, an increase in pregnancy rate was observed as compared with the blastocyst on day 6.^[12,13] These results can be explained as because of delayed embryo growth as well as displaced window of implantation when day 6 embryos are transferred in fresh ET cycle. Shapiro et al. and Khorram et al. have shown that embryos at expanded blastocyst stage and transferred on day 5 have high implantation potential compared to those for which expansion and transfer was done on day 6.^[13,14]

In frozen cycle, there are conflicting results of effect of vitrified good-quality day 5 and day 6 blastocyst on pregnancy outcome. Systemic review and meta-analysis of 15 controlled studies^[6] comprising slow-freezing and vitrification methods of cryopreservation reported higher clinical pregnancy rate and ongoing pregnancy rate for day 5 transfer compared to day 6 transfers. However, further analysis of these studies where day 5 and day 6 blastocysts had the same morphological quality at the time of freezing showed no difference in clinical pregnancy and ongoing pregnancy rates. Our study too did not find any statistical significance in terms of clinical and biochemical pregnancy rate on day 5 and day 6. Recent meta-analysis by Yi‐Xin Li et al. observed lower implantation rate and lower clinical pregnancy rate in day 6 blastocyst-stage embryo transfer.^[15]

However, these findings are not similar in case of SET. Kaye et al. as authors observed no difference in ongoing pregnancy rate, clinical pregnancy rate, live birth rate, or miscarriage rate between day 5 and day 6 frozen and vitrified embryos.^[16] Similar to this study, our results also illustrate no significant difference in clinical pregnancy rate, ongoing pregnancy rate, implantation rate, and miscarriage rate in groups of day 5 and day 6.

Literature evidence showed that one of the important reasons for implantation failure is embryo aneuploidy. Recently, Sardana et al.^[17] have demonstrated that in PGT-A cycle, irrespective of day of transfer no statistically significant difference was observed in live birth rate following transfer of euploid embryos, whereas day 5 blastocyst transfer was associated with higher live birth rate compared to day 6 blastocysts in untested embryos. In our study, all the embryos were PGT embryos, raising an important question if these cycles would perform better if PGT-A is offered.

Our study shows that day 6 vitrified blastocysts lead to higher miscarriage rate compared to day 5 blastocysts. Similar results were obtained by Xu et al., where they found that day 6 blastocysts transfer were associated with a higher early miscarriage rate.^[18] This raises an important question if these cycles would perform better if preimplantation genetic testing is offered where only day 6 embryos are available for transfer.

CONCLUSION

Initial pregnancy potential of high-grade blastocysts frozen on day 5 or day 6 after in-vitro fertilization and replaced in FET donor recipient cycles seems to be comparable. In donor recipient cycles having younger age of donors, SET of day 6 cryopreserved blastocysts resulted in similar clinical pregnancy rate, though significantly higher miscarriage rate.

Ethics Compliance: All ethical approvals required for the study were obtained before the start of the study from Ethical Committee of Sir Ganga Ram Hospital.