If you think your baby has a fever, it’s a good idea to check it with a thermometer rather than feeling his forehead. A digital thermometer can be a good choice, but ask your doctor whether to take your baby’s rectal (in the bottom), oral (in the mouth) or axillary (under the arm) temperature, based on his age. Do you have a mercury thermometer in the house? The American Academy of Pediatrics says not to use it and to remove it from your home to prevent accidental mercury exposure.

 

Source:
"How to take a Child's Temperature", http://www.healthychildren.org/English/health-issues/conditions/fever/pages/How-to-Take-a-Childs-Temperature.aspx