During the first 20 years of IVF embryo transfers were performed on days 1 to 3, even though evidence suggested that embryos dispelled into the uterus from fallopian tubes were at a post-compaction stage. This meant that initially all embryo transfers were asynchronous transfers. This strategy was imposed upon IVF by the vitro culture conditions of the time, which were sub-optimal for normal rates of blastocyst development. Transferred asynchronously cleavage stage embryos have to adapt to "alien" non- physio-metabolic conditions that may result in embryonic stress and compromised embryo development. In fresh embryo transfer this is further exacerbated by the supraphysiological hormone effects of controlled ovarian stimulation. In addition, there are mechanical factors (i.e. uterine contractility) and time factors, related to the endometrium (i.e., window of implantation) and embryo (i.e., time to hatching), that may reduce the chances of implantation. In contrast, to most other mammalian species the transfer of cleavage stage embryos to human uteruses led to acceptable levels of pregnancy and live birth.