As we move to reducing the number of embryos transferred in a given IVF cycle, ideally down to one, there is an ever-increasing need for noninvasive quantitative markers of embryo viability. Although stage-specific morphologic markers and grading systems have been developed, such an approach is unable to assess the physiological status of the embryo. Analysis of metabolism has proved to be a valuable marker of embryo viability after transfer in animal models. We therefore reviewed what is known about human embryo metabolism, how media systems can affect the patterns of nutrient utilization and the activities of metabolic pathways, and how this relates to the developmental competence of the embryo. It is proposed that a unifying hypothesis of metabolism for the entire preimplantation period is not realistic, given the dramatic changes in embryo physiology that occur from fertilization to blastocyst development, and that the concept of a "quiet metabolism" can be interpreted as stress induced by the presence of high oxygen in the embryo culture/analysis system. Further research is required to fully understand the origins of metabolic stress in embryos for it to be alleviated and to develop a comprehensive range of markers that not only reflect embryo viability, but also sex-specific differences in physiology.
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