PGS, PGD, and Their New Names
Preimplantation genetic testing of embryos to be used in IVF is a fairly recent innovation in fertility treatment. Most people will not need genetic testing as part of their IVF treatment, but it can be very helpful to patients whose families have inherited genetic diseases, women who are older, and people who have experienced recurrent pregnancy loss.
These tests are designed to improve the chances of a successful IVF cycle by selecting chromosomally normal or “euploid” embryos. Recently, the terminology has changed in the U.S. to be more reflective of what the tests screen for and to be uniform around the world. As the name implies, the embryos are tested before implantation in the uterus, and are an extra step in the IVF process. Just a few cells are biopsied from the embryo for testing.
PGS, Now PGT-A
Preimplantation genetic screening (PGS) screens for the correct number of chromosomes. Too many or too few chromosomes, known as aneuploidies, are leading causes of failure to implant and of miscarriages, and are more likely to happen as a woman ages. This test does not screen for specific genetic diseases, although it can detect Down syndrome, which is caused by the presence of an extra chromosome 21. The new name of this test is PGT-A, which means Preimplantation Genetic Testing for Aneuploidies. Your fertility specialist may recommend this test if you are 35 or over and having IVF treatment with your own eggs, or if you have had recurrent miscarriages. Costs include the cost of the cell biopsy at the embryology lab, usually around $1,500, and the cost charged by the genetic testing company, usually $1,000 – $2,000.
PGD, Now PGT-M and PGT-SR
Preimplantation genetic diagnosis (PGD) screens for specific inherited abnormalities, either for inherited genetic diseases or for inherited chromosomal rearrangements. PGT-M is an abbreviation for Preimplantation Genetic Testing for Monogenic/Single Gene Disorders, and screens for specific inherited conditions controlled by a single gene. These include Huntington’s disease, Tay-Sachs disease, cystic fibrosis, sickle cell disease, and Fragile X syndrome.
PGT-SR tests for chromosomal structural rearrangements (SR), like translocations or inversions. This occurs when a portion of chromosomes break and re-fuse in a different location. If the pieces rearrange so that no chromosomes are lost or gained, it’s known as a balanced translocation. About one in 500 people have a balanced translocation and do not suffer any health consequences. An unbalanced translocation means there is extra genetic material, or not enough. Translocations are a major cause of miscarriage. A child with an unbalanced translocation may have learning disabilities, developmental delay, or health problems. The severity of the issues depend on where the unbalanced translocation occurs.
These tests are carried out on different platforms and are generally more expensive than PGT-A. PGT-M is extremely important to consider before IVF if you or your male partner have a family history of an inherited disease. If you have had recurrent miscarriages, your reproductive endocrinologist may recommend karyotype testing for both you and your partner. Karyotyping determines the number of chromosomes you each have and looks for missing or translocated chromosomes. If one of you has a balanced translocation, the chances of your embryos having an unbalanced translocation are increased and your fertility specialist may recommend PGT-SR.