During the last three decades, there has been a major increase in the understanding of embryo physiology and correspondingly major improvements in the in vitro culture technologies used, in particular culture media and incubator technologies. These improvements have resulted in the culture of more good quality cleavage stage embryos and consequently blastocysts after extended culture, increasing embryo utility and the pregnancy capacity of an cycle. The increased implantation rates obtained with blastocyst transfers may be as much to do with embryo quality and stage, i.e., blastocysts have the required bi-directional communication competency needed for implantation, as with the WOI, i.e., the time of blastocyst formation and transfer coincides with the WOI of endometria. Moreover, the presently improved culture conditions may also be changing traditional embryo developmental milestones, in particular, the time to blastulation may be decreasing. An increasing number of blastocysts are being formed on day 4 of culture - ±100 hours post-insemination. Traditionally, day 4 embryos were only assessed for evidence of compaction, which is the developmental stage leading to blastulation and cell differentiation.