A fever is generally defined as a temperature of 38°C (100.4°F) or above when measured accurately. Readings vary slightly by method: rectal and forehead (temporal) measurements tend to run a little higher than under-the-arm (axillary) readings, and the most reliable method depends on your child's age. For babies under 3 months, a rectal reading is often recommended for accuracy.
Fever itself is not an illness; it is the body's normal response to infection, usually viral. A higher number does not automatically mean a more serious illness. A child who is alert, drinking, and playing with a temperature of 39°C is often less concerning than a listless child with a lower fever.
For a baby under 3 months with any fever of 38°C or above, contact a doctor promptly, as fevers in very young infants are treated more cautiously. For older children, watch how they look and behave overall, and speak to a doctor or pharmacist if you are unsure.