Embryos undergoing delayed second mitosis exhibit a reduced blastocyst formation rate and a lower live birth even if third mitosis was completed on time

Background: The phenomenon of day3 (D3) 8-cell embryos derived from day2 (D2) 2-cell embryos can occur in in vitro fertilization (IVF) laboratories. Abnormal cleavage patterns can predict delayed second mitosis. Here, we investigated the developmental potential of these embryos to provide a reference for the selection of embryos for transplantation. Methods: Embryos were divided into four groups according to cell number on D2 and D3. Group A featured high-quality D2 2-cell embryos that developed into high-quality D3 8-cell embryos; group B featured the same type of embryos as group A except the embryos developed to compaction; group C featured high-quality D2 4-cell embryos that developed into D3 8-cell high quality embryos and group D featured the same type of embryos as group C except the embryos developed to compaction. Results: The blastocyst formation rate of group A and group B was significantly lower than that of group C ( P<0.001 ). With regards to D3 single embryo transfer (SET) and frozen single blastocyst transfer (SBT), the clinical pregnancy and live birth rates of group A were comparable with those of group C. For D3 double-embryo transfer (DET), the biochemical pregnancy rate, clinical pregnancy rate, and live birth rate, following the combined transfer of embryos from group A and Group C was significantly lower than that when transferring two embryos from group C. Conclusions: Embryos with delayed second mitosis exhibited a lower blastocyst formation rate and poorer embryo outcomes even though third mitosis was completed on time. Blastocyst culture is useful for selecting blastocysts with a higher probability of live birth for embryos with delayed second mitosis. Trial registration: This study is retrospective and does not require trial registration.