Comparison of pregnancy and live birth rates between fresh day 5 morula transfer and fresh day 6 blastocyst transfer following extended culture for slow growing embryos

Purpose The aim of this study is to evaluate and compare the outcomes of fresh day 5 morula transfers and fresh day 6 blastocyst transfers after extended culture, in women with a whole cohort of slow growing embryos. Methods Fresh embryo transfer cycles of patients that underwent intracytoplasmic sperm injection between 2013 and 2020 with a whole cohort of slow developing embryos on 5th day of fertilization were evaluated for this retrospective cohort study. Outcomes of day 5 morula transfers and day 6 blastocyst transfers after extended culture were compared. Results Out of 479 patients, day 5 morula transfers were performed to 194 and embryo culturing was extended to day 6 in 285 women. Blastocyst formation was observed in 129 of 285 patients in extended culture group. Implantation rate of day 6 blastocyst transfer group was significantly higher than day 5 morula transfer group (27% vs. 12%, p < 0.001). However, clinical pregnancy (17% vs 15.4%) and live birth rates (14.4% vs 13%) were found similar in day 5 morula transfer and extended culture group per initiated cycle. Conclusion Although implantation capacity of day 6 blastocysts seems higher comparing to day 5 morulas, clinical pregnancy and live birth rates are similar among intention to treat population in fresh day 5 morula transfers and day 6 blastocyst transfers after extended culture for women that cohorts consist of only slow developing embryos.

Automated summary

This study evaluated and compared the outcomes of fresh day 5 morula transfers and fresh day 6 blastocyst transfers after extended culture in women with a whole cohort of slow developing embryos. The results showed that although the implantation rate of day 6 blastocyst transfers was significantly higher, the clinical pregnancy and live birth rates were similar between the two groups. Therefore, for women with only slow developing embryos, there is no significant difference in the success rates between fresh day 5 morula transfers and day 6 blastocyst transfers after extended culture.