Pre-implantation Genetic Diagnosis (PGD) was the term used to describe any genetic tests performed on embryos for chromosomal abnormalities and genetic diseases. It had since been changed to Pre-implantation Genetic Screening (PGS), then to the current term Pre-implantation Genetic Testing (PGT) which is divided into three subtypes, PGT-A, PGT-M, PGT-SR.
PGT-A (Pre-implantation Genetic Testing for Aneuploidy) is the screening of embryos by counting the 46 chromosomes to look for extra or missing chromosomes (“aneuploidy”). PGT-A is used to select embryos that are the most likely to implant and result in a successful pregnancy. It also reduces the chance of having a child with extra chromosomes, such as Downs syndrome.
PGT-M (PGT for Monogenic or single gene disorder) is used to help patients reduce their risk to have a child with a known inherited genetic disorder caused by mutations in a single gene (“monogenic”), such as cystic fibrosis or hearing loss.
PGT-SR (PGT for Structural Rearrangement) is for hereditary chromosome abnormalities whereby one or both parents have a balanced chromosome “structural rearrangement” (such as translocations or inversions). PGT-SR reduces the risk of having a pregnancy or child with an unbalanced structural abnormality, which involves extra or missing genetic material and typically results in a pregnancy loss.
PGT-A is the most common type of genetic testing being performed. For the most part, embryos undergoing genetic testing are frozen while awaiting test results. A single, genetically determined normal, embryo is selected for a frozen embryo transfer on the patients succeeding cycle. The advantages of transferring genetically normal embryo include higher probability of pregnancy (60-70% compared to around 40% for non-tested high quality embryo), lower miscarriage rate, less chance of multiple gestations, and the ability to select a certain gender for family balancing. Some of the disadvantages are high cost, additional cycle is needed to perform embryo transfer because embryos have to be frozen, and the possibility that none of the embryos are suitable for embryo transfer if they all are determined to be abnormal.