A reliable technique to identify the embryos with the greatest chance for implantation is fundamental in any IVF laboratory. Indiscriminate transfer of multiple embryos in order to improve outcome increases the likelihood of multiple gestations and leads to a higher rate of obstetrical complications. However, todate embryo morphology remains the most widely accepted and single best predictor of embryo implantation. Morphologic evaluation of embryos is based on a grading system that of embryos with the greatest probability for implantation. The determination of embryo morphology on the number and symmetry of blastomeres and the percentage of extracellular fragmentation.  
 
While human embryos are routinely transferred 2-3 days after fertilization at 4-8 cell stage, mammalian data suggest that IVF/ET may be more efficient if embryos are transferred at the blastocyst stage.  
 
Culture media is presumably a source for growth factors and proteins necessary for embryo in vitro.