Male advantage observed for in vitro fertilization mouse embryos exhibiting early cleavage

PURPOSE: Mouse embryos forming blastocoele early vs those forming late are predominantly male. We examined whether the male advantage could be recognized at an earlier stage of development. METHODS: The IVF embryos were classified into early, intermediate, and late development groups based on the time of the third cleavage, and the Zfy and Xist genes were detected to identify their sex in the classified embryos. Furthermore, embryos that were classified based on the time of the third cleavage were transferred to recipient animals and the sex ratio of the fetuses was determined at birth. RESULTS: Approximately 90% of the early‐developing embryos that exhibited third cleavage as early as 47 hours after insemination were male when analyzed using PCR at the blastocyst stage. PCR analysis showed that 61% of the intermediate‐developing embryos (third cleavage occurring 48‐50 hours after insemination) and 45% of late‐developing embryos (third cleavage occurring at 51 hours or later postinsemination) were male. After embryo transfer, the early‐developing embryos produced 80% males, while intermediate‐ and late‐developing embryos produced 56% and 45% males, respectively. CONCLUSIONS: Male embryos tend to develop faster than female embryos during early stage of preimplantation in mice.