Stem cell-based embryo models have emerged as powerful scientific and ethical alternatives to the use of embryos in research. Nicolas Rivron led the development of the first complete embryo model. It reflects the blastocyst in mice (2018, Nature) and humans (2021, Nature) and is named blastoid. Blastoids are unique in two respects: they are complete and thus allow the study of embryogenesis as a whole, and they represent the pre-implantation stage thereby enabling implantation in utero and ultimately the study of all subsequent stages (e.g. gastrulation, organogenesis). Using blastoids, the Rivron lab explores (1) the conserved principles of embryonic self-organization, focusing on the coordination and pacing of molecular and mechanical signalings in the context of human heterochronies, and (2) the human genetic traits that have led to our low fecundity compared to other primates (50% of fertilized oocytes never develop). This fundamental research aims to uncover the evolutionary constraints and vulnerabilities of human embryogenesis, knowledge of which is necessary for the development of novel therapeutic approaches to global health challenges such as family planning, fertility decline and prenatal preventive medicine. Nicolas Rivron is co-founder of dawn-bio, a biotech company that leverages blastoids for human reproductive medicine. These advances have ethical implications. Nicolas Rivron is contributing to drawing a roadmap for the gradual, justifiable use of embryo models, and for the dissemination of a sensible, accurate picture of contemporary human embryology that aim to maximize public support and societal benefit. Some of these activities have been carried out within the framework of the International Society for Stem Cell Research (2018-2021), national ethics committees (UK parliament & G-SCBEM, France, Netherlands, Germany, Scandinavia, EU consortium HYBRIDA; 2021-today) and the EMBL Ethics Board (2024-today).