The number of mitochondrial copy number has a huge variation among oocytes from the same cohort and also from oocytes from different patients. It is believed that this initial number of mtDNA content is correlated with the ability of oocytes to fertilized and to achieve blastocyst stage. Whatever the initial concentration of mtDNA a given oocyte will have, it expected to equally segregate into the number of blastomeres along embryo development, so the mtDNA copy number of per daughter cell will transiently decrease, being the smallest at the blastocyst stage. Studies in different mammalian species including humans have shown that mtDNA replication does not occur before morula stage so the mitochondrial content of the oocytes should be enough to sustain embryo development before implantation occurs.