Focusing on the case of time-lapse imaging (TLI), this paper analyses how medical professionals negotiate the use of new ‘add-on’ fertility treatments in light of the limited evidence available. The data produced by TLI technologies is meant to help professionals identify the best embryo to be implanted. Embryo selection is essential in IVF practice for increasing pregnancy rates and reducing the negative effects of repeated failures. More than 5 years after the introduction of TLI in IVF labs, however, there has been no conclusive randomised control trial (RCT) evidence to show that the tools do indeed have a significant impact on pregnancy rates. Nonetheless, many public clinics in the UK have adopted such technologies. Consequently, our research asks: How is the use of TLI tools legitimised by professionals, in light of contradictory evidence? Focusing on 25 semi-structured staff interviews, we argue that professionals use several strategies to legitimise the use of TLI in the clinic without, however, challenging the tenets of evidence-based medicine (EBM) and the value it places on RCTs. Rather, professionals emphasise various advantages that TLI offers, including its use as a lab tool, its potential for knowledge production in embryology, and the role it plays in the management of patient expectations and course of treatment. This paper contributes to debates on the role of EBM in modern medicine and fertility care specifically – an area where this inter-relationship has been underexplored. We conclude by suggesting avenues towards a more nuanced understanding of EBM as it relates to IVF treatment and a rapidly changing biotechnology context.Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.