Hypoxic–ischaemic insults occurring during or after birth can cause both acute and long‐term neurological impairment. The duration of the insult is a critical factor, but most published reports of duration have important limitations. After the onset of a persistent bradycardia in 125 term born infants, abnormal outcomes occurred in two by 10 minutes, in 12 out of 47 (26%) delivered between 11 and 20 minutes, and in 55 out of 65 (85%) delivered after 20 minutes. Series with unspecified gestation or including infants born preterm give comparable results in over 500 additional cases. Before 20 minutes there was little correlation with severity, while after 20 minutes most were severely impaired. Limited neuroimaging data suggest that damage restricted to the basal ganglia and thalamus may begin to occur after 10 minutes, associated Rolandic damage after 15 minutes, and other cortical involvement after 20 minutes. Associated white matter damage can occur after any duration. There were little data for other patterns of damage.What this paper adds Some term born infants can withstand 20 minutes of fetal bradycardia without acute or chronic damage. Durations in humans are not the same as in animal models.