preimplantation genetic diagnosis (PGD) The screening of early embryos for disease-causing genes to enable the selection of ‘healthy’ embryos. The technique is used in conjunction with in vitro fertilization, which typically yields a number of embryos. A single cell is removed from an eight-stage embryo and subjected to genetic testing; for example, it may be tested for a specific disease allele using either a gene probe and fluorescence in situ hybridization or the polymerase chain reaction. If the results are satisfactory, the embryo is implanted in the mother's uterus, and development proceeds. Removal of a single cell at this stage does not affect the embryo's subsequent development. PGD can help especially when couples who are being treated for fertility problems also have a history of genetic disease. However, use of PGD can be extended in nontherapeutic ways, such as choosing a baby's sex or selecting particular desired traits to produce so-called ‘designer babies’. These highly controversial applications of PGD are prohibited in certain countries, including the UK.

preimplantation genetic diagnosis (PGD) (pree-im-plahn-tay-shŏn) n. prenatal genetic diagnosis extended to the earliest stages of embryonic development, before implantation occurs.