A study reveals that preimplantation genetic testing for aneuploidy, a standard IVF procedure, cannot detect certain genetic changes that occur in embryos shortly before implantation. Researchers observed these abnormalities in real time using advanced microscopy on thawed human embryos. While the findings highlight limitations in embryo screening, their impact on pregnancy success remains unclear.
In vitro fertilisation (IVF) involves genetic testing of embryos before transfer to the uterus to identify chromosomal issues that could lead to miscarriage or conditions like Down syndrome. The common test, known as preimplantation genetic testing for aneuploidy (PGT-A), is performed about 5 to 6 days after fertilisation. It entails removing a few cells from the embryo's outer layer to check for extra or missing chromosomes. However, this provides only a snapshot, as cells continue dividing before implantation, which typically occurs 1 to 5 days after transfer.
Ahmed Abdelbaki at the University of Cambridge and his colleagues investigated these post-testing changes by monitoring 13 thawed human embryos for 46 hours, simulating the period between screening and implantation. Using a light-sheet microscope to minimize light exposure—unlike conventional methods limited to 24 hours—they injected a fluorescent dye that binds to DNA. Across 223 dividing cells, they found that 8 per cent experienced chromosome misalignment during division, increasing the risk of aneuploidy that could hinder implantation or cause developmental issues.
These errors were limited to the outer layer of cells, which forms the placenta, and did not affect the inner cells that develop into the fetus. Previous studies indicate that embryos with such outer-layer abnormalities can still lead to successful pregnancies, suggesting these changes may not always compromise viability, according to Abdelbaki.
"There may be later [genetic] changes in the embryo after the point at which we are screening with PGT-A," says Lilli Zimmerman at Northwell Health in New York state. She adds, "What this study, to me, really shows is that there is still a lot more research needed in terms of screening embryos for whether [they] are genetically normal or abnormal." Zimmerman notes the small sample size limits broader applicability, and the effects on embryo viability are uncertain.
The research was published in Nature Biotechnology (DOI: 10.1038/s41587-025-02851-1).