Fever in A Newborn

Adults have a tightly controlled thermostat to help keep their body temperature at a normal level. When you are cold, You shiver, helping to raise your body temperature. When you are overheated, you sweat to help you cool off. These mechanisms are not completely developed in newborns. Newborns also lack the insulating fat layer that older babies and children have.

Because a newborn's temperature regulation system is immature, your baby may not have a fever with infection or illness. Fever in babies can be caused by other, more serious things. Call your baby's doctor immediately if your baby is younger than 3 months old and has a rectal temperature of 100.4°F (38°C) or higher. 

Infection

Fever is a normal response to infection in adults, but only about half of newborns with an infection have a fever. Some, especially premature babies, may have a lowered body temperature with infection or other signs such as a change in behavior, feeding, or color.

Overheating

While it is important to keep a baby from becoming chilled, a baby can also become overheated with many layers of clothing and blankets. This can occur at home, near heaters, or near heat vents. It can also occur when a baby is overbundled in a heated car. Avoid placing a baby in direct sunlight, even through a window. Never leave a baby in a hot car even for a minute. The temperature can rise quickly and cause heat stroke and death.
An overheated baby may have a hot, red, or flushed face, and may be restless. To prevent overheating, keep rooms at a normal temperature, about 72°F  to 75°F (22.2°C to 24° C), and dress your baby just like you and others in the room.

Low fluid intake or dehydration

Some babies may not take in enough fluids which causes a rise in body temperature. This may happen around the second or third day after birth. If fluids are not replaced with increased feedings, dehydration (excessive loss of body water) can develop and cause serious complications. Intravenous (IV) fluids may be needed to treat dehydration.