If you’re going through IVF, you may be offered a test to look at your embryos’ chromosomes. Pre-implantation genetic testing for aneuploidy (chromosome abnormalities), known as PGT-A, is an “add on” used to help choose embryos with the right number of chromosomes. It’s promoted by IVF clinics as a way to increase the chance of success, especially for women over 35. But the evidence shows that in most cases, PGT-A doesn’t improve the chance of a baby. What is aneuploidy? Human cells usually contain 46 chromosomes. Aneuploidy is a term that describes a chromosome number that is different from 46 – either too many or too few chromosomes. In human embryos, most aneuploidies are lethal, resulting in miscarriage, or do not result in pregnancy at all. The chance of aneuploidy increases with the age of the woman; by the time a woman reaches age 40, approximately 80% of her embryos are aneuploid.