Researchers using optimized long‑term light‑sheet imaging of human preimplantation embryos observed de novo mitotic chromosome segregation errors at blastocyst stages, findings reported in Nature Biotechnology. The studies show that some chromosomal abnormalities arise after the typical window when preimplantation genetic testing for aneuploidy (PGT‑A) is performed, raising questions about the test’s timing and interpretation. Authors caution that live imaging reveals dynamic mosaicism in trophectoderm cells and advise reassessment of clinical PGT‑A workflows and counseling practices.