Science influences ethics and policy-making. For example, a high incidence of embryo loss in the earliest stage of pregnancy is commonly used as a contributory justification for the intentional or foreseen loss of human embryos in fertility treatment and laboratory experimentation. However, establishing the natural fate of embryos during the first week post-fertilisation is challenging, and hampered by a lack of appropriate data. Many scientific sources are claimed to justify embryo mortality rates of 75% and higher. These include speculative calculations, demographic analyses, biochemical data and the unique anatomical studies of Dr Arthur Hertig. However, a critical re-evaluation of these data casts doubt on quantitative conclusions that are often repeated, and occasionally exaggerated, by both scientists and ethicists. In this talk Dr Gavin Jarvis, a pharmacologist from the University of Cambridge, will introduce and present those studies which provide the evidential basis for claims about early human embryo mortality, and highlight the effects that a misunderstanding of this issue may have on ethics and science alike.