 All biopsied embryos from the test and control group will have their mitochondrial DNA analyzed, but that information will not be used for purposes of choosing embryos for replacement. Retrospectively but blindly see blinding of results section, the information will be used at the end of the study to determine which embryos have a higher chance of implanting. If at that point the participating patients have remaining embryos frozen, they will be able to use that information for purposes of embryo selection. 