Objective To inform the assessment of described mechanisms of bruising in children.

Design Prospective cross-sectional study.

Setting The emergency department, and children in the local community.

Patients Children aged 0–13 years with bruises from unintentional injuries. Exclusions: bleeding disorder, medication affecting coagulation or child protection concerns.

Interventions Injury incidents were categorised into one of eight causal mechanisms (fall from<1 m, 1–2 m, fall from standing height or less and hitting an object during fall, stairs or impact, crush, sports or motor vehicle collision).

Main outcome measures Location, number and mechanism of bruising for each injury mechanism.

Results 372 children had 559 injury incidents, resulting in 693 bruises; 85.2% of children were walking independently, with impact injuries and fall from standing height (including hitting an object) being the predominant mechanisms. A single bruise was observed in 81.7% of all incidents. Stair falls resulted in ≥3 bruises only with falls involving ≥10 steps (6/16). Bruising was rarely observed on the buttocks, upper arm, back of legs or feet. No bruises were seen in this dataset on ears, neck or genitalia. Petechial bruising was only noted in 1/293 unintentional incidents, involving a high-impact injury in a school-aged child.

Conclusion These findings have the potential to aid an assessment of the plausibility of the explanation given for a child with bruising. Certain bruise distributions were rarely observed, namely multiple bruises from a single mechanism, petechiae and bruising to the ears, neck or genitalia.