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What’s the most accurate way to take a baby’s temperature?
Q: When the doctor tells a parent to call if an infant under 3 months has a fever of 100.4 degrees is that based on an oral, axillary or rectal reading? I have found that they all measure a little differently.
A: Cris, thanks for your question about temperatures and fevers. It can be confusing with the different ways of measuring a child’s temperature and the different thresholds for concern depending on the age of a child. The definition of a fever for an infant under 3 months of age is 100.4 degrees Fahrenheit or 38 degrees Celsius measured rectally (in the baby’s bottom). This is because the rectal temperature is considered the most accurate for infants.

In addition to taking infants’ temperatures rectally, you can also take their temperature by the axillary (underarm) method or with an ear thermumeter. Some ear thermumeters have different settings to read out the temperature as if it were taken orally or rectally. Generally, the closer the thermumeter is to the inside of the body, the warmer the temperature it records. Usually, the axillary temperature is the lowest, an oral temperature is 1 degree Fahrenheit higher, and the rectal temperature is 1 degree higher than that. For example, the axillary temperature of 99 degrees is equivalent to an oral temperature of 100 degrees and a rectal temperature of 101.

If your baby is under 3 months of age and has a rectal temperature of 101 degrees or higher, you should call your doctor. Infants’ health is far more delicate than that of older children, and we should never take chances with them. The doctor will ask you questions about how your baby is feeding and acting, and may need to examine her to make sure she doesn’t have a serious illness.

In fact, doctors differ in their opinions about fevers and when you should call. Be sure to talk with your own doctor to know what he or she recommends.

Karen Sokal-Gutierrez M.D., M.P.H. Pediatrician