Maternal clinical deterioration can usually be identified by monitoring and charting key vital signs (e.g. respiratory rate, oxygen saturation (SpO2), blood pressure (BP)).1 Paper-based charts remain the principal means of documenting vital signs, despite a drive to introduce electronic automated systems.2 These charts are intended to track maternal physiology temporally, highlight vital sign deviations and ‘signpost’ the need for a prompt clinical response. They employ a range of approaches to improve the recognition of abnormalphysiology.3 Some charts use colour-coded shading, often yellow and red, to highlight ‘mildly’ and ‘markedly’ abnormal parameter ranges.4 Others incorporate an aggregate early warning system (EWS) score that allocates points to each vital sign measurement based on the extent of their derangement from predefined ‘normal’ ranges.5