Thermometer basics: Taking your child's temperature

The best way to take your child's temperature depends on his or her age and ability to cooperate. Understand thermometer options — and when to seek medical help for a fever.

If your child feels warm or seems under the weather, you should probably take his or her temperature. Sounds simple enough — but if you're new to it, you probably have a few questions. Which type of thermometer should you use? Is an armpit temperature good enough? Are the thermometer rules different for babies and older children? Here are answers to these questions and more.

Know your thermometer options

Years ago, a glass mercury thermometer was a staple in most medicine cabinets. Today, mercury thermometers have largely been replaced by digital thermometers.

Regular digital thermometers use electronic heat sensors to record body temperature, often in 30 seconds or less. Regular digital thermometers can be used in the mouth, armpit or rectum. Digital ear thermometers, also called tympanic thermometers, use an infrared ray to measure the temperature inside the ear canal. They're even quicker than regular digital thermometers, often measuring body temperature in just a few seconds. A digital pacifier thermometer is another option for young kids.

If you want to invest in a single thermometer for the entire family, a regular digital thermometer is probably best. If you'd like the option to use an ear thermometer or a pacifier thermometer, you might want to keep one of each on hand, too.

Age matters, too

The best type of thermometer — or the best place to insert the thermometer, in some cases — depends on your child's age.

Birth to 3 months. For newborns, use a regular digital thermometer to take a rectal temperature. Lubricate the tip of the digital thermometer with petroleum jelly. Lay your baby on his or her back, lift your baby's thighs, and insert the lubricated thermometer one inch into your baby's rectum. Stop if you feel any resistance. Hold the thermometer in place until it beeps.
3 months to 4 years. For older infants and toddlers, you can use a digital ear thermometer or a digital pacifier thermometer. Carefully follow the instructions that came with your thermometer. You can also use a regular digital thermometer to take a rectal temperature or an armpit temperature. When you place the thermometer under your child's armpit, make sure it touches skin — not clothing. Hold the thermometer in place until it beeps.
4 years and older. By age 4, most kids can hold a digital thermometer under the tongue for the short time it takes to get a temperature reading. Place the tip of the thermometer under your child's tongue and ask your child to keep his or her lips closed. Remove the thermometer when it beeps. If your child has been eating or drinking, wait at least 20 minutes to take his or her temperature by mouth. If your child is too congested to breathe through his or her nose, you may need to take an armpit or rectal temperature — or use a digital ear thermometer.

Accuracy varies

Rectal temperatures are the most accurate, especially for young children. Temperatures measured by mouth and by ear also are accurate, if done properly. Armpit temperatures and temperatures measured with a pacifier thermometer are the least accurate.

Whatever the method, make sure you know exactly how to use your thermometer. Read the instructions that came with your thermometer. After each use, clean the thermometer with rubbing alcohol or soap and lukewarm water. For safety — and to make sure the thermometer stays in place — never leave your child unattended while you're taking his or her temperature.

When to seek medical advice

A fever is a common sign of illness, but that's not necessarily a bad thing. In fact, fevers seem to play a key role in fighting infections. If your child is drinking plenty of fluids, sleeping well and continuing to play, there's usually no reason to treat a fever.

If you want to give your child medication to treat a fever, stick to acetaminophen (Tylenol, others) until age 6 months. If your child is age 6 months or older, ibuprofen (Advil, Motrin, others) is OK, too. Read the label carefully for proper dosage. Don't give aspirin to anyone age 18 or younger.

In general, contact your child's doctor if your child:

Is younger than age 3 months and has a rectal temperature of 100.4 F (38 C) or higher
Is older than age 3 months, has a temperature up to 102 F (38.9 C) and seems unusually irritable, lethargic or uncomfortable
Is older than age 3 months and has a temperature of 102 F (38.9 C) or higher that doesn't respond to over-the-counter medication

When reporting a temperature to your child's doctor, give the actual reading and state how the temperature was taken. Don't add or subtract numbers from the reading depending on where the thermometer was placed.