Parents can use many different types of thermometers, including oral, rectal, ear, forehead, and more. Here's how the different methods measure up.

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A fever indicates that your child's body is trying to fight off some kind of infection. If your little one has a fever, they may be warm to the touch, flushed or sweating, or have chills. Because being bundled in too many clothes or blankets may result in fever-like symptoms, taking your child's temperature is the best and most reliable way to determine whether a fever is present.

Knowing how to use a thermometer to measure your child's temperature is important. Temperatures can be taken in several different places on the body with various thermometers; whichever type of thermometer you use, be sure to read the directions carefully. Almost all thermometers require correct positioning to get the most accurate reading.

Although many fevers run their course without medical intervention, see your doctor if your child's temperature is higher than 102 degrees Fahrenheit for more than 24 hours or if the fever is higher than 104 degrees F. For infants under 3 months, call the doctor if the fever is over 100.4 degrees F.

When you report a fever to your doctor, always tell them where you took your child's temperature and which type of thermometer you used. Here's how different thermometers measure up.

Rectal Thermometer

Because it's the most internal and therefore the most accurate measurement, many doctors recommend taking a rectal temperature in babies and children 3 years of age and younger. To use, first dab petroleum jelly on the bulb of the clean thermometer. Place your baby belly-down on your lap or changing table. You can also position your baby face-up on the changing table, lifting their legs (knees to chest) as though you were changing a diaper. Then gently insert the bulb 1/2 to 1 inch into your child's rectum. Loosely hold the thermometer in place with two fingers until it beeps, making sure your baby doesn't squirm too much.

Oral Thermometer

Placing the thermometer under the tongue in cooperative children over 3 is also accurate, provided the child keeps their mouth closed and refrains from drinking hot or cold liquids 15 to 20 minutes before the thermometer is inserted. The reading can be thrown off if your child doesn't keep the thermometer under their tongue until it beeps.

Ear (Tympanic) Thermometer

Tympanic thermometers can also gauge fever in just a few seconds, but they're expensive—most run about $60—and they 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. For a more precise result, pull your child's ear slightly up and back before inserting.

Underarm Thermometer

Your child's armpit gives the least accurate reading. However, using a thermometer under the arm for four to five minutes is still good in a pinch, especially if your child doesn't let you insert one anywhere else. Many doctors recommend adding 1 degree F to the reading for greater accuracy, because the temperature under the arm is generally lower than the temperature at the core of the body. If the room is chilly, you may get a false low reading.

Digital Forehead (Temporal) Thermometer

These are fast, fairly accurate, and great to use when your baby is too young to sit still. Make sure that the infrared sensor is near the temporal artery (between the outside edge of the eye and the hairline). Look at the directions carefully to make sure you get an accurate reading.

Wearable Thermometer 

Wearable thermometers can be used continuously, and they don't require waking or disturbing the child. Their ability to track the fever across time can be helpful. Most wearable thermometers connect to an app to display the temperature reading. According to The Washington Post, accuracy may vary, because wearable thermometers usually measure skin temperature instead of core temperature. Other factors can also influence the reading, such as location of the sensor, physical exertion level, and outside temperature.

Forehead Thermometer Strips 

While these aren't as accurate as thermometers, they are the least invasive and disruptive way to take a child's temperature. Simply hold the strip on the forehead until the color registers along the strip.

NOTE: No matter which type of thermometer you choose, you should consider ditching old-school varieties that contain mercury, a potent toxin that affects the brain, spinal cord, liver, and kidneys, and can cause learning disabilities. If it breaks, you risk exposing your family to mercury's harmful vapors. Still have one lurking in your medicine cabinet? Don't just toss it into the trash. Take it to your pediatrician (they can dispose of it safely), or drop it off at your local hazardous-waste collection site.