Teams using light‑sheet live imaging tracked late‑stage human preimplantation embryos and found de novo mitotic chromosome segregation errors in cells destined for the trophectoderm, raising questions about the timing and interpretation of preimplantation genetic testing for aneuploidy (PGT‑A). The Nature Biotechnology papers describe methods to label nuclei and follow divisions in blastocysts, demonstrating that some chromosomal errors arise after the stage when clinical PGT‑A samples are taken. Authors call for caution in relying solely on single‑timepoint trophectoderm biopsies and suggest imaging and temporal data could refine embryo selection.