In order for an embryo to implant and progress to be a normal pregnancy, it requires to contain the correct number of genetic material, known as chromosomes. PGT-A (also known as PGS) is the process of screening an embryo diagnosing chromosomal alterations (called aneuploidy) before it is transferred in to the uterus. Chromosomally normal embryos both have a higher chance of implantation, and the resulting pregnancies have a lower chance of miscarriage. Before PGT-A can be used in routine practice it requires further research, however, it might be appropriate in certain cases.
For more information about genetic testing, see here