Pregnancy is defined as a 'red flag' in headache assessment. We aimed to describe the prevalence and causes of serious secondary headache in pregnant ED patients.Unplanned secondary analysis of HEAD Study/HEAD Colombia data.3.2% (117/3643) of ED headache patients aged 18-50 years were pregnant, of whom six (5.1%) had a serious secondary cause identified. The proportion of patients with serious headache causes was not significantly different between pregnant female, non-pregnant female and male patient subgroups (P = 0.89).Inclusion of pregnancy as a 'red flag' in ED headache assessment is not supported by these data.