Evaporation dominates heat loss during skin barrier maturation in extremely preterm infants: a prospective longitudinal mixed-effects analysis

Background Extremely preterm infants exhibit immature skin barrier function, resulting in excessive water and heat loss. However, the temporal dynamics and mechanisms of heat loss remain unclear. Methods We conducted a prospective longitudinal study of infants born at 22–28 weeks of gestation ( n = 30). Transepidermal water loss (TEWL), insensible water loss from the skin (IWLs), and heat loss components were measured from birth to 28 days. Linear mixed-effects models were applied across four gestational age groups, adjusting for time-varying environmental conditions as management-related covariates. Results IWLs was significantly higher in lower gestational age groups and decreased nonlinearly over time. Maturation rates differed across groups. Evaporative heat loss consistently accounted for the majority of total heat loss across all gestational age groups. Environmental factors were associated with outcomes but reflected protocol-driven management rather than independent physiological determinants. Conclusion Skin barrier maturation follows a gestational age–dependent trajectory, and evaporative heat loss appears to be a major contributor to heat loss. Early evaporative control may play an important role in thermal management of extremely preterm infants. These findings suggest that both skin immaturity and environmental management factors contribute to thermal instability in this population, highlighting evaporation as a physiological pathway during early postnatal adaptation. Impact This study provides the first daily serial assessment of TEWL and IWLs in infants born at 22–28 weeks’ gestation. It enables the derivation of total heat loss on a day-by-day basis from direct physiological measurements. The findings further quantify the daily contribution of evaporative heat loss to total heat loss. These results offer new insights into heat loss dynamics and support targeted evaporative control in neonatal care.