What PGT stands for
PGT stands for preimplantation genetic testing — "preimplantation" because it happens before the embryo is placed in the uterus, "genetic testing" because it examines the embryo's chromosomes or genes. You may also see the older term PGD (preimplantation genetic diagnosis), and two subtypes: PGT-A (A for aneuploidy — checking the chromosome count is correct) and PGT-M (M for monogenic — checking for one specific inherited condition).
PGT is done on a blastocyst before transfer: an embryologist removes a few cells from the outer layer (the trophectoderm, not the part that becomes the baby), and the biopsy is sent for genetic analysis while the embryo itself is frozen and waits for the result. The main use, PGT-A, screens for the correct number of chromosomes (aneuploidy screening); PGT-M tests for a specific inherited condition already known to run in the family.
What it changes, and what it doesn't
PGT helps choose which tested-normal embryo to transfer first — it doesn't create better embryos, and a normal result doesn't make pregnancy or a healthy baby certain, since implantation depends on more than chromosomes. It's discussed most often for recurrent implantation failure, advanced maternal age, or a known genetic condition — not as a routine add-on for every cycle. See PGT: who actually benefits for the fuller picture.