Preimplantation Genetic Testing for Aneuploidy (PGT-A) is a laboratory technique used alongside In Vitro Fertilization (IVF) to screen embryos for an abnormal number of chromosomes before they are transferred to the uterus. A small number of cells are biopsied from the trophectoderm (the outer layer of a day 5–6 blastocyst, a cluster of dividing cells that forms five to six days after a sperm fertilizes an egg) and analyzed using next-generation sequencing. Embryos are then classified as chromosomally normal (euploid), abnormal (aneuploid), or mosaic (a mix of normal and abnormal cell lines).