The most accurate way to measure body temperature is to use a rectal thermometer. Measuring rectally is a bit of a hassle, but unlike an oral- or forehead temperature measurement, a rectal temperature measurement can not be influenced by outside factors like drinking, eating or walking outside on a cold day.
It is also the recommended way to check the temperature of your newborn. In your baby’s first few months, a fever can indicate a possibly serious infection and is a reason to call your doctor immediately.
Body temperature varies slightly throughout the day. Body temperature is at its highest in the late afternoon. Make sure to check your baby’s temperature multiple times a day if they are running a fever.
Properly taking rectal temperature measurements
Clean the probe (the pointed end) before and after use with rubbing alcohol or soap and rinse it with a damp cloth. Or use a probe cover for even better hygiene. Do not use cold or hot water to clean the thermometer, as this can cause the thermometer to give an inaccurate reading.
Wipe a small amount of petroleum jelly on the end of the probe.
Put your baby on their back and hold their legs up with their knees at stomach height. Make sure they are secure and do not wiggle too much.
Turn on the thermometer and carefully insert the thermometer half an inch into the rectum.
Gently hold the thermometer until it beeps, then take it out carefully and read the temperature.
Clean the thermometer again. If you used a probe cover, remove it and throw it away.
Should I call a doctor?
Any temperature above 97 Fahrenheit (°F) or 36.1 Celsius (°C) and under 100.4°F (38°C) is considered a normal body temperature.
For a baby younger than 3 months always call your doctor when their temperature is 100.4°F (38°C) or higher.
If your baby is between 3 and 6 months old you should call a doctor when their fever reaches 102°F (38.9°C) or they seem sick with a fever between 100.4°F (38°C) and 102°F (38.9°C).
If your baby is 6 to 24 months old contact a doctor when their temperature is higher than 102°F (38.9°C) and does not decrease after a day. Also contact your doctor if they show other signs or symptoms like a cold, a cough or diarrhea.
Never use a rectal thermometer to check your temperature orally. Make sure to keep your thermometers separated and code them with colors or letters.
Do you want to know how to take oral temperature more accurately? Check out our “How to use an oral thermometer” blog!