New optimized live‑imaging methods for human preimplantation embryos revealed chromosome segregation errors arising de novo in cells that later populate the placenta, according to two complementary reports in Nature Biotechnology. Researchers applied long‑term labeling and imaging of cultured embryos to visualize mitotic mistakes during blastocyst development and to map where errors occur within cell lineages. The lead findings show that some aneuploidies emerge in placenta-fated lineages rather than in embryo-fated cells, complicating the interpretation of preimplantation genetic tests and placental sampling. Authors emphasized that the techniques provide a new window into early human developmental biology and carry potential implications for assisted reproductive technologies and embryo selection.