Support and regulation of the thermal environment of the newborn have long been recognized as critical aspects of newborn care, and they have become increasingly important as smaller and less mature infants are able to survive. In this review, the foundational work done more than 50 years ago that defined the impact of hypothermia on morbidity and mortality in infants is discussed, and the concept of the neutral thermal environment is described, as well as how the identification of a narrow range of body temperature in which metabolic and oxygen demands are at their lowest has ensured safety in infant care while facilitating growth and optimal outcome. Thermoregulation is discussed within the framework of a balance between heat production and loss. The unique physiologic mechanisms that are available to the newborn to reduce loss and produce extra heat when facing a cold stress are described. The relatively large amount of brown adipose tissue in the newborn is a key energy source for heat production, although the metabolic processes and control mechanisms surrounding nonshivering thermogenesis differ between the more vulnerable premature infant and the term infant. In contrast to the means of heat production, the capacity of the newborn for self-protection against heat loss is limited. Without external support, newborns can readily lose heat and body temperature through all four mechanisms of heat loss, including evaporation, conduction, radiation, and convection, although each plays a greater or lesser role at various times after birth. Unless measures are taken to minimize these losses, severe physiologic derangements may result.
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