The most accurate way to take an infant’s temperature is to use a digital thermometer rectally or orally. Make sure you carefully read the instructions that came with your thermometer.
To take his temperature rectally, lubricate the tip of the thermometer with petroleum jelly. Place your baby on his back, lift your baby’s thighs and insert the lubricated thermometer ½ to 1 inch (1.3 to 2.5 centimeters) into the baby’s rectum. If you feel any resistance stop immediately. Hold the thermometer in place for about a minute or until the thermometer signals that it’s done. Remove the thermometer, and read the number.
If your infant is 3 months or younger and has a rectal temperature of 100.4°F or higher, call your doctor or go to the emergency room immediately.