Objective It is essential that paediatricians assessing a child with bruising in suspected physical abuse can identify when the explanation for the bruise is implausible. Several published studies describe bruising patterns in populations of abused and non-abused children but we are not aware of any that describe the relationship between mechanism of injury and bruise pattern.

Methods Details of the mechanism of unintentional injury, the number of bruises, location and size were recorded for: children aged 0–11 years attending a Children’s Emergency Department (ED) at a tertiary centre and children in the community (4–11 years at school, 0–4 year olds at two nurseries) with bruises of known cause. Students T-test, Chi-squared and Fisher’s exact test were used to analyse associations between mechanisms and bruise patterns.

Results 351 children were recruited. Nine mechanisms of injury were recorded (Table 1). 81% (p < 0.001) of non-ambulant children (mean age = 9.9 months, SD = 6.3) sustained impact injuries whilst 60% of ambulant children (mean age = 41.7 months, SD = 30.4) fell from <1m or standing height (p < 0.001). The majority of children presented with small (mean size = 17.4mm, range = 2–115mm (ED), 1–75mm (Community)), and single (86.9%) bruises related to each incident.

Falls from <1m, falls from standing height hitting an object and impact injuries accounted for 79.5% (279/351) of all incidents. Bruises were most commonly found on the forehead (n = 114, 29.7%) knees and shins (n = 88, 22.9%) and head (n = 39, 10.2%) whilst bruising to certain areas were rare e.g. the front trunk (1.3%), back of thigh (0.9%) and genitalia (0.3%).

Conclusion The data demonstrate predominant injury mechanisms for unintentional injury in children that relate to child’s level of development. Multiple bruises from a single incident were rare, bruises were small and predominantly to the forehead, knees and shins and head. This predictable pattern of unintentional bruising can aid clinicians to judge the likelihood that a pattern of bruising is consistent with the mechanism of injury, and improve the detection of physical abuse where bruising pattern does NOT fit with a proposed mechanism.