Fetal and neonatal thermoregulation.

The metabolic rate of the fetus per tissue weight is relatively high when compared to that of an adult. Moreover, heat is transferred to the fetus via the placenta and the uterus, resulting in a 0.3 degrees C to 0.5 degrees C higher temperature than that of the mother. Therefore, fetal temperature is maternally dependent until birth. At birth, the neonate rapidly cools in response to the relatively cold extrauterine environment. Thus, the neonatal temperature rapidly drops soon after birth. In order to survive, the neonate must accelerate heat production via nonshivering thermogenesis (NST), which is coupled to lypolysis in brown adipose tissue. Heat is produced by uncoupling ATP synthesis via the oxidation of fatty acids in the mitochondria, utilizing uncoupled protein. Thermogenesis must begin shortly after birth and continue for several hours. Since thermogenesis requires adequate oxygenation, a distressed neonate with hypoxemia cannot produce an adequate amount of heat to increase its temperature. In contrast to the neonate, the fetus cannot produce extra heat production. This is because the fetus is exposed to inhibitors to NST, which are produced in the placenta and then enter the fetal circulation. The important inhibitors include adenosine and prostaglandin E2, both of which have strong anti-lypolytic actions. The inhibitors play an important role in the metabolic adaptation of a physiological hypoxic fetus because NST requires adequate oxygenation. Furthermore, the presence of NST inhibitors allows the fetus to accumulate an adequate amount of brown adipose tissue before birth. The umbilical circulation transfers 85% of the heat produced by the fetus to the maternal circulation. The remaining 15% is dissipated through the fetal skin to the amnion, and is then transferred through the uterine wall to the maternal abdomen. As long as fetal heat production and loss are appropriately balanced, the temperature differential between the fetus and the mother remains constant (heat clump). However, when the umbilical circulation is occluded for any reason, the fetal temperature will rise in relation to the extent of the occlusion. The fetal temperature may elevate to the hyperthermic range in cases of acute cord occlusion; if this occurs, fetal growth, including brain development, may be impacted. Experimentally induced cord occlusion, which is recognized as a significant cause of brain damage, results in a rapid elevation of body temperature; however, the brain temperature tends to remain constant. This is considered to be a cerebral thermoregulatory adaptation to hypoxemia, which has the physiologic advantage of protecting the fetus from hyperthermia, a condition that predisposes the fetus to hypoxic injury (cerebral hypometabolism). A number of thermoregularatory mechanisms are in place to maintain normal fetal and neonatal growth. Data has primarily been collected from animal studies; aside from the strict thermal control provided in the newborn nursery, little information exists concerning these mechanisms in the human fetus and neonate. Probably further information on thermoregulation is necessary specially to improve perinatal management for hypoxic fetuses.