The “14‐day rule”—broadly construed—is used in science policy and regulation to limit research on human embryos to a maximum period of 14 days after their creation or to the equivalent stage of development that is normally attributed to a 14‐day‐old embryo (Hyun et al, 2016; Nuffield Council on Bioethics, 2017). For several decades, the 14‐day rule has been a shining example of how science policy and regulation can be developed with interdisciplinary consensus and applied across a number of countries to help fulfil an ethical and practical purpose: to facilitate efficient and ethical embryo research. However, advances in embryology and biomedical research have led to suggestions that the 14‐day rule is no longer adequate (Deglincerti et al, 2016; Shahbazi et al, 2016; Hurlbut et al, 2017). Therefore, should the 14‐day rule be extended and, if so, where should we draw a new line for permissible embryo research? Here, we provide scientific, regulatory and ethical arguments that the 14‐day rule should be extended to 28 days (or the developmental equivalent stage of a 28‐day‐old embryo).