Pre-implantation genetic testing (PGT) is a new method in the field of assisted reproduction. The evolution started with technology that enabled single-cell screening of chromosomes, and developed in resolution with molecular approaches, to the extent that specific genetic sequences (gene mutations) can be identified in such samples. Hence screening for whole chromosomes (aneuploidy) developed into molecular definitions (from specific mutations to entire screens) in embryos. However, embryology and its understanding, revealed that mosaicism of abnormal lineages is relatively common; this impacted on how abnormal these embryos are, and whether these embryos should be transferred. An even newer possibility is the use of cell-free DNA from human embryo spent culture media for diagnosis, and it will be a guiding and reference value for clinic practice.