For decades, IVF relied on relatively crude methods for assessing embryos: either visual inspection of embryo morphology or genetic probes. These methods targeted only some chromosomes and necessarily reduced embryonic viability into a binary outcome: ‘normal’ or ‘abnormal’. The introduction of next-generation sequencing technologies in the mid 2010s disrupted this neat interpretive framework. Rooted in the principles of precision medicine, an approach that seeks to tailor healthcare decisions to each patient’s unique genetic, molecular, and environmental profile – these technologies offered a far more granular view of the embryo’s genome. The newfound visibility of mosaicism destabilised established decision-making frameworks and raised urgent practical and ethical questions. This chapter uses the evolving guidelines issued by the Preimplantation Genetic Diagnosis International Society (PGDIS) and the American Society for Reproductive Medicine (ASRM) on mosaic embryos as central reference points and sites of narration, with a view to exploring how the same body of evidence can be interpreted/narrated in divergent and at times conflicting ways, both in formal recommendations and in terms of public uptake. Divergent interpretations raise broader questions in reproductive medicine, including what types of knowledge are recognised as evidence and what role technological advances play in compelling us to reconsider concepts as fundamental as normality and abnormality.