Preimplantation genetic diagnosis (PGD) is an ART procedure in which genetic testing is done on biopsied cell(s) from a preimplantation-stage embryo. With this technology, only unaffected embryos are transferred back to the recipient uterus to initiate a pregnancy. PGD was originally developed to screen for single gene disorders but is currently used to detect different factors, including HLA-type, chromosome abnormalities, and aneuploidy. Genetic testing can be done on biopsies of the first or second polar bodies, a single blastomere from a cleavage-stage embryo, or several trophectoderm cells from a blastocyst.