Because it's the most internal and therefore the most accurate measurement, many doctors recommend taking a rectal temperature -- for two to three minutes with a mercury thermometer -- in babies and children 3 years of age and younger.
Placing the thermometer under the tongue in cooperative children over 3 is also accurate, provided the child keeps his mouth closed for two to three minutes and refrains from drinking hot or cold liquids 15 to 20 minutes before the thermometer is inserted. Should you use a digital or a mercury thermometer here? Whichever you prefer. (Some studies say digitals are as on-the-mark as mercury; other studies find them to be slightly less so.)
Tympanic thermometers can also give you a good gauge of a fever in just a few seconds, but they're expensive -- most run about $60 -- and require batteries. If you don't insert them just right into your child's ear canal or there's a buildup of earwax, they can also be inaccurate.
This spot gives the least accurate reading. However, using a mercury thermometer under the arm for four to five minutes is still good in a pinch, especially if your child refuses to allow you to insert one anywhere else.