Objective The use of therapeutic hypothermia (TH) for mild hypoxic–ischaemic encephalopathy (HIE) remains controversial and inconsistent. We analysed trends in TH and maternal and infant characteristics associated with short-term outcomes of infants with mild HIE.

Design Retrospective cohort analysis of the California Perinatal Quality Care Collaborative database 2010–2018. E-value analysis was conducted to determine the potential impact of unmeasured confounding.

Setting California neonatal intensive care units.

Patients 1364 neonates with mild HIE.

Interventions Supportive care versus TH.

Main outcome measures Factors associated with TH and mortality.

Results The proportion of infants receiving TH increased from 46% in 2010 to 79% in 2018. TH was more likely in the setting of singleton birth (OR 2.69, 95% CI 1.21 to 5.39), no major birth defects (OR 2.18, 95% CI 1.42 to 3.30), operative vaginal delivery (OR 3.04, 95% CI 1.80 to 5.10) and 5-minute Apgar score ≤5 (OR 3.17, 95% CI 2.43 to 4.13). Mortality was associated with small for gestational age (OR 5.79, 95% CI 1.90 to 18.48), <38 weeks’ gestation (OR 7.31 95% CI 2.39 to 24.93), major birth defects (OR 11.62, 95% CI 3.97 to 38.00), inhaled nitric oxide (OR 12.73, 95% CI 4.00 to 44.53) and nosocomial infection (OR 7.98, 95% CI 1.15 to 47.03). E-value analyses suggest that unmeasured confounding may have contributed to some of the observed effects.

Conclusions Variation in management of mild HIE persists, but therapeutic drift has become more prevalent over time. Further studies are needed to assess long-term outcomes alongside resource utilisation to inform evidence-based practice.