Your first reaction when you suspect a fever is probably to feel your child's forehead. Most moms quickly become expert at recognizing a temperature this way, but you still need to use a thermometer for an exact reading. The type of thermometer you should use depends on your child's age. What not to use: an old-fashioned mercury thermometer. If the thermometer breaks, your child can be exposed to or poisoned by the toxic mercury inside. Fortunately, digital electronic thermometers have become the normand they're much easier to use, with some models giving readings in under a minute.

For the most accurate readings in babies under three months of age, the American Academy of Pediatrics considers a rectal temperature to be the gold standard. Your pediatrician will likely insist you use this method if your baby is six months or younger. After that, you can start using an underarm, ear, or forehead (temporal-artery) thermometer, but your doctor may ask you to confirm that reading with a rectal temperature if he is concerned, up until about a year of age. Always let your pediatrician know what type of thermometer you used, so he can take that into consideration.

Once your child is about 5 years of age, you should be able to use a standard oral thermometer. For oral readings, allow 20 minutes after a child is finished eating or drinking hot or cold foods and beverages before taking her temperature. Also, never take a child’s temperature right after a bath. The reading will not be accurate.

How to Take a Rectal Temperature

• Coat the end of a clean thermometer (preferably a digital one with a round bulb at the end) with petroleum jelly

• Lay your baby on her back and lift her legs up in the air

• Glide the thermometer about a half-inch into the rectum (stop if you feel any resistance)

• Gently hold the thermometer in place until it beeps or gives a signal that it's done.