Adults have a more sophisticated body 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. Newborns also lack the insulating fat layer that older babies and children develop.  

Because a newborn's temperature regulation system is immature, fever may or may not occur with infection or illness. However, fever in babies can be due to other causes that may be even more serious.  

Call your baby's doctor right away if your baby younger than 3 months old has a rectal temperature of 100.4° F or higher.  The best and most accurate way to check your baby’s temperature is rectally. 

Fever in newborns may be due to: