High‑resolution, light‑sheet live imaging of late‑stage human preimplantation embryos revealed de novo mitotic errors in trophectoderm cells that were not detectable at earlier stages, challenging assumptions behind preimplantation genetic testing for aneuploidy (PGT‑A). The findings were published in Nature Biotechnology and highlighted by Cambridge researchers. The teams fluorescently labeled nuclei and observed embryos for extended periods, showing chromosome segregation errors that can arise after the stage at which PGT‑A biopsies are commonly taken. Authors caution that PGT‑A may overestimate embryo aneuploidy when later mitotic events produce mosaic patterns restricted to placenta‑fated cells. Clinics, regulators and ethicists will need to weigh how live‑imaging insights affect embryo selection protocols and counseling. The work does not prescribe immediate clinical changes but signals the need for further research to align testing windows with developmental error dynamics.