The best way to take a temperature in infants and young children is by measuring the temperature in the rectum. This is done by gently inserting the tip of a thermometer through the opening in your baby's bottom where stool (poop) comes out.
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Use a digital thermometer labeled for rectal use. Do not use a glass thermometer that contains mercury, as these aren't safe. Label the rectal thermometer so it won't be used for oral temperatures.
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If your baby had a bath or has been bundled tightly for a while, wait at least 20 to 30 minutes to take the temperature.
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Before and after use: Clean the end of the thermometer with rubbing alcohol or soap and water and rinse with cool water.
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Coat the tip of the thermometer with a lubricant, such as petroleum jelly, and turn it on.
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Place your baby:
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belly-down across your lap or on a firm, flat surface, and keep your palm along the lower back; or
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face-up with legs bent toward the chest with your hand against the back of the thighs
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With your other hand, insert the lubricated thermometer into the anal opening about ½ inch to 1 inch (about 1.25 to 2.5 centimeters), or until the tip of the thermometer is fully in the rectum. Note the angle of the thermometer in the picture above. Stop if you feel any resistance.
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Steady the thermometer between your second and third fingers as you cup your hand against your baby's bottom. Speak quietly to soothe your child.
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When the thermometer signals that the temperature is ready to be read, remove it gently.
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Write down the number on the thermometer screen and the time of day.
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A reading of 100.4°F (38°C) or above means your baby has a fever.
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If your baby is 3 months old or younger, a reading of 97°F (36.1°C) or below means your baby has an abnormally low temperature.