Preimplantation genetic diagnosis

Preimplantation genetic diagnosis (PGD or PIGD) is the genetic profiling of embryos prior to implantation (as a form of embryo profiling), and sometimes even of oocytes prior to fertilization. PGD is considered in a similar fashion to prenatal diagnosis. When used to screen for a specific genetic disease, its main advantage is that it avoids selective abortion, as the method makes it highly likely that the baby will be free of the disease under consideration. PGD thus is an adjunct to assisted reproductive technology, and requires in vitro fertilization (IVF) to obtain oocytes or embryos for evaluation. Embryos are generally obtained through blastomere or blastocyst biopsy. The latter technique has proved to be less deleterious for the embryo, therefore it is advisable to perform the biopsy around day 5 or 6 of development. The world's first PGD was performed by Handyside, Kontogianni and Winston at the Hammersmith Hospital in London. "Female embryos were selectively transferred in five couples at risk of X-linked disease, resulting in two twin and one singleton pregnancy." The term preimplantation genetic screening (PGS) refers to the set of techniques for testing whether embryos (obtained through IVF/ ICSI have an abnormal number of chromosomes (aneuploidy). PGS is also called aneuploidy screening. PGS was renamed preimplantation genetic diagnosis for aneuploidy (PGD-A) by the Preimplantation Genetic Diagnosis International Society (PGDIS) in 2016. The PGD allows studying the DNA of eggs or embryos to select those that carry certain mutations for genetic diseases. It is useful when there are previous chromosomal or genetic disorders in the family and within the context of in vitro fertilization programs. These kinds of tests are needed because hereditary disorders are related with 20% of child deaths in developed countries. It is estimated that, as a whole, they are responsible for around 18% of pediatric hospital admissions. The procedures may also be called "preimplantation genetic profiling" to adapt to the fact that they are sometimes used on oocytes or embryos prior to implantation for other reasons than diagnosis or screening. Procedures performed on sex cells before fertilization may instead be referred to as methods of oocyte selection or sperm selection, although the methods and aims partly overlap with PGD.