Adults have a tightly controlled internal thermostat to help regulate their body temperature. When cold, an adult shivers, helping to raise the temperature of the body. Sweating occurs when an adult is overheated, to allow for cooling. These mechanisms, on the other hand, are not completely developed in newborns. In addition, newborns lack the insulating fat layer that older babies and children develop.
Because a newborn's temperature regulation system is immature, she may not get a fever when fighting an infection or illness. However, fever in babies can be due to other causes, which may be more serious. Call your baby's physician immediately if your baby younger than 3 months old has a rectal temperature of 100.4 degrees F or higher.