The Rules for Taking a Baby's Armpit Temperature

When you are taking baby's temperature, you want to do it right. Taking an axillary temperature -- under the armpit -- is more time-consuming and often less accurate than taking a rectal temperature. For very young babies, the rectal method may be preferred for accuracy. But axillary temperatures can provide an adequate screening tool when you use the correct thermometer and follow some simple rules for achieving an accurate result.

Choosing the Correct Thermometer

To take an axillary temperature, use a digital thermometer. Either an oral or rectal digital thermometer will work, but designate the thermometer for axillary use only, to avoid contamination. Ear thermometers and digital thermometers that scan the temples or forehead are available for purchase, but they won't work in the armpit. The old-style glass and mercury thermometers, no longer sold in the United States, should not be used because of the risk of injury from broken glass and from the mercury inside.

Placing the Thermometer

Normal Body Temperature for Children

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To take an accurate axillary temperature, the thermometer point must fit snugly into your baby's armpit. If you don't get a snug fit, the reading you get will probably be too low. Make sure that your baby's clothing doesn't get between the thermometer and the skin. The baby's skin should completely surround the thermometer. Removing your baby's clothing will make it easier. Dry the armpit, since moisture conducts heat and may give a false reading. Place the thermometer as high up into the armpit as possible, with the tip pointing toward your baby's head. Hold your baby's arm down against his side to ensure that the tip of the thermometer is surrounded by skin.

Waiting Enough Time for an Accurate Reading

It may take a concerted effort to hold a thermometer in your infant's armpit for the length of time needed. According to an April 2006 study published in "Archives of Disease in Childhood," most digital thermometers will register within 40 to 80 seconds when taking an axillary temperature, and beep to let you know when to read it. If you get a reading that seems unusually low or high, try taking the temperature under the other armpit for a comparison, or using a different digital thermometer.

Determining Whether Your Baby Has a Fever

Differences Between Taking a Child's Temperature Orally and Under the Arm

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An axillary temperature will normally be lower than a rectal temperature. A common definition for fever using the rectal method is 100.4 F, while for the axillary method it's 99.3 F. A systematic review published in April 2000 in "BMJ" found that there was wide variation between axillary and rectal temperatures. Rectal temperatures are the "gold standard" for accurate measurement. Call your doctor whenever your baby appears ill, whether or not a fever registers on an axillary thermometer reading.