The human embryo is unique among mammalian embryos, in that within its cells, a high proportion have an incorrect number of chromosomes. Human cells should have 22 paired chromosomes plus two sex chromosomes (either XX or XY). But many studies have revealed that at the start of life, only about a 1/5th of embryos at Day 5 of development contain this normal pattern in all cells. Yet surprisingly, there is also a tolerance to the level of ‘aneuploidy’, the term used to describe embryos with an incorrect number of chromosomes in cells. Human embryos are susceptible to aneuploidy for reasons that no-one fully understands, but it is related to parental age, particularly the mother’s age; health and environmental factors. Current methods to detect aneuploidy requires an embryo biopsy, where a small number of cells are removed from the surface of the embryo using micro-manipulation techniques. This small sample of cells are then genetically assessed for the number of chromosomes in each biopsied cell. Biopsy itself is a highly invasive technique, known to negatively impact ongoing embryo health.
Many clinics offer embryo aneuploidy screening (termed Preimplantation Genetic Testing – (for) Aneuploidy, or just PGT-A), but it is controversial, as analysis of the literature suggests that at most, it assists preventing miscarriage in older women. One deficiency is that current biopsy techniques can only sample a few cells of the embryo. At Day 5 of development, there are two types of cells – the ‘outside’ cells (trophectoderm) that form the placenta and the inside disk of cells (inner cell mass cells) that are the progenitor cells for the developing fetus. Because the biopsy technique only takes a small sample of the ‘outside’ cells, these may not reflect accurately the state of aneuploidy that exists across the whole embryo. Added to this, as with other embryo selection technologies, PGT-A does not improve the quality of embryos produced.