When is a baby's fever an emergency?

The rule every parent should memorize: under 3 months old, a rectal temperature of 100.4 F (38 C) or higher is a fever, and the AAP says that means calling the doctor right away - day or night. Young babies can have serious infections without looking that sick, and their immune systems cannot be given the benefit of the doubt. Expect the advice to be an exam the same day, sometimes the ER. From 3 to 6 months, the threshold relaxes slightly - call promptly for temperatures around 102 F, or lower fevers with an unusually fussy or sleepy baby. Over 6 months, behavior becomes the guide: a feverish baby who still drinks, plays a little, and makes eye contact can usually be watched at home, while one who is hard to wake, refusing fluids, breathing fast, or having a seizure needs urgent care at any temperature. Measurement matters. Rectal is the reliable method for infants - the AAP notes it is the most accurate for babies. Forehead and ear thermometers are convenient but can read low, so a borderline reading in a young baby deserves a rectal check. And fever in a baby who has been in a hot car or bundled heavily can be heat-related, which is its own emergency. Pymander is a free AI doctor, 24/7 by text. It can walk you through the age-based rules while you take the temperature and help you decide where to go.