Assessment of embryo viability: the ability to select a single embryo for transfer--a review.

By being able to select the most viable embryo(s) within a given cohort it will be possible to reduce the number of embryos transferred in a given IVF procedure. Several morphological scoring systems have been proposed for the successive stages of human embryo development. Other indicators of embryo viability include rate of cleavage. Finally, non-invasive methods of assessing nutrient uptake and utilization have been developed that can be used to measure the health of individual embryos. A sequential scoring system has therefore been proposed that uses the above parameters in order to create a history for each embryo during the preimplantation period. Such systems will help lead to single embryo transfers for the majority of IVF patients.