The system that controls body temperature is not well developed in a newborn baby. Call your baby's doctor right away if your baby is younger than 3 months old and has a rectal or forehead (temporal) temperature of 100.4°F (38°C) or higher. This is an emergency. You will need to take your baby to the closest emergency room (ER) for assessment.

Use a digital thermometer to check your child's temperature. Don't use a mercury thermometer. There are different kinds and uses of digital thermometers. The types you can use on a newborn baby are:

• Rectal. For children younger than 3 years, a rectal temperature is the most accurate.
• Armpit (axillary). This is the least reliable but may be used for a first pass to check a child of any age with signs of illness. The doctor may want to confirm with a rectal temperature.
• Forehead (temporal). This works for children age 3 months and older. If a child under 3 months old has signs of illness, this can be used for a first pass. The doctor may want to confirm with a rectal temperature.

Use the rectal thermometer with care. Follow the product maker's directions for correct use. Insert it gently. Label it and make sure it's not used in the mouth. It may pass on germs from the stool. If you don't feel okay using a rectal thermometer, ask the doctor what type to use instead. When you talk with any doctor about your child's fever, tell them which type you used.

Below are guidelines to know if your young child has a fever. Your child's doctor may give you different numbers for your child. Follow your doctor's specific instructions.

Fever readings for a baby under 3 months old:

• First, ask your child's doctor how you should take the temperature.
• Rectal or forehead: 100.4°F (38°C) or higher
• Armpit: 99°F (37.2°C) or higher

A fever is common when an adult has an infection. In newborns, fever may or may not occur with an infection. A newborn may actually have a low body temperature with an infection. They may also have changes in activity, feeding, or skin color.

It's important to keep a baby from becoming chilled. But a baby can also become overheated with many layers of clothing and blankets. An overheated baby may have a hot, red, or flushed face, and may be restless. To prevent overheating:

• Keep your baby away from any source of heat. For example, a room heater, fireplace, heating vent, or direct sunlight.
• Keep your home at about 72°F to 75°F.
• Dress your baby comfortably. A baby doesn't need more clothing than you do.
• Cars can get very hot. Be extra careful when dressing your baby to go for a car ride.

Newborns may not take in enough breast milk or formula. This may cause an increase in body temperature. If you think your baby isn't eating enough of either breast milk or formula, call the doctor. Make sure you know how to check your baby's temperature and have a thermometer. Call your baby's doctor right away if your baby has a fever.

Contact your baby's doctor right away if your baby is younger than 3 months old and has a rectal temperature of 100.4°F (38°C) or higher. This is an emergency. You will need to take your baby to the closest emergency room (ER) for assessment.