Defining fever in children

Well, fever is defined as a rectal temperature that is above 38 degrees Celsius (100.4 degrees Fahrenheit).
Hyperpyrexia (extremely high fever), is defined as a temperature that is above 40 degrees Celsius (104 Fahrenheit). Is a high fever as such necessarily more concerning? Read more about this in a dedicted chapter.

What is considered normal temperature in children?

Normal temperatures in children vary throughout the day (the range is usually between 36.6-37.9 degrees Celsius). Bodily temperature is usually higher in the evening than in the morning (there is usually about half a degree difference between the two timings).

On a practical note, I am often asked whether a temperature of 37.6 in a child means he is healthy and nothing ‘iffy’ is happening. I am also often asked about a temperature of 37.9. If the temperature isn’t above 38 – does that mean the child is not ill?

We must remind ourselves that the reasoning behind defining fever as a temperature that is above 38 degrees Celsius was made for us to both have a general medical definition as well as for research purposes. Fever can described as a spectrum of temperatures and there is no doubt that repeated temperatures of 37.9 are also abnormal and may indicate underlying illness.

Generally speaking, one can say that a temperature below 38 degrees Celsius in a child above two months of age usually does not indicate a significant acute (present) illness.
Nonetheless, temperatures that are near 38 degrees, even if they are not above 38, require a professional assessment by a pediatrician, especially in infants under the age of two months where the general approach towards fever may be stricter (read more here).