Human embryo viability: what determines developmental potential, and can it be assessed?

The understanding of the embryo's nutrient requirements and physiology has led to the development of more physiological culture media, capable of supporting acceptable levels of human blastocyst development in vitro. The success of such media can be attributed to catering to the embryo's changing nutrient requirements, while minimizing culture-induced stress, thereby facilitating normal cell function. Most important, blastocysts derived from such sequential culture systems have a high viability. The ability to identify those blastocysts from within a given cohort which have the highest developmental potential will lead to further increases in implantation and pregnancy rate. Such an approach should ultimately lead to the routine transfer of a single blastocyst in a given IVF cycle, while being able to maintain a high pregnancy rate.