PGT-A (Preimplantation Genetic Testing for Aneuploidy) is genetic screening of embryos for chromosomal abnormalities. It involves biopsy of embryos reaching the blastocyst stage during In Vitro Fertilization (IVF) to determine which embryos have normal chromosome number and structure.

PGT-M (Preimplantation Genetic Testing for Monogenic disease) is preimplantation genetic testing of embryos to detect hereditary single gene (monogenic) defects, such as cystic fibrosis (CF). The technique decreases the risk of transmitting hereditary disease to the children. PGT-M requires biopsy of the embryos to select normal or unaffected embryos for transfer during IVF treatment.

Typically, during IVF, 1 or 2 embryos are transferred into the uterus 5-7 days after fertilization at the blastocyst stage. Not all embryos progress to the blastocyst stage and not all blastocysts are genetically normal. Selecting the right embryo for transfer is tricky because looks can be deceiving. The embryo may look good and still be genetically abnormal.

Remarkably, about 40% of chromosomally abnormal embryos can reach the blastocyst stage. Not surprisingly, when a single non-biopsied blastocyst is transferred, the ongoing pregnancy rate in patients under the age of 35 is only about 40%. Transfer of two non-biopsied embryos results in 55% ongoing pregnancy rate.

Chromosome abnormalities (aneuploidy) in human embryos is the major cause of in vitro fertilization (IVF) failure and miscarriage. The incidence of chromosomal abnormalities in embryos increases exponentially in women over the age of 35 years.

Data from embryo biopsy demonstrates that the incidence of genetically abnormal embryos increases from 30% to 50% in patients under 35 years of age to 80% in women 42 years of age or older. This explains why live birth rates per IVF egg-retrieval cycle decrease from 55% in young patients (under 35) to about 15% in women aged 41–42 years.