Updated June 2026 · 90-second read + video
A fever (100.4°F / 38°C or higher) in a baby under 3 months is always an ER visit — no exceptions. For older babies, the number matters less than how they look: feeding, alert, and consolable usually means home care; lethargy, trouble breathing, or won't drink means go.
The 90-second version
The fever rules I use for my own kids and in clinic.
What to watch for
For fever, age and behavior matter far more than the number on the thermometer.
Any rectal temp 100.4°F (38°C) or higher = ER, every time.
Hard to rouse, no wet diapers, refusing all fluids.
Fast or hard breathing, blue lips, or a seizure — call 911.
I don't treat the number, I treat the kid. A 104 fever in a giggling toddler worries me less than a 101 fever in a baby who won't wake up.
The way I'd explain it over coffee. Tap any question.
A newborn's immune system isn't tuned up yet, and they can't show you they're sick the way an older child can. A fever can be the only sign of a serious bacterial infection — so we work it up every time, no exceptions.
Fever is the body's response to infection — it's a tool, not a disease. The brain raises the thermostat to make the body a less friendly place for viruses and bacteria. It's why we focus on how your child looks, not the exact number.
Both work. Tylenol (acetaminophen) is okay from any age; Motrin (ibuprofen) is only for babies 6 months and older. Dose by weight, not age, and never combine without your doctor's okay.
Quick answers to what parents ask most.
100.4°F (38°C) by any reliable method. Rectal is the gold standard in infants; armpit/forehead readings often run a degree low.
A brief whole-body seizure triggered by a fast-rising fever, usually in kids 6 months to 5 years. Scary to watch, almost always harmless, but a first one should always be evaluated.
Under 3 months with any fever = ER. Anyone with trouble breathing, dehydration, or a seizure = ER. Otherwise an older, stable child with fever can usually start with a pediatrician or urgent care.
Any fever lasting more than 5 days, or one that goes away and comes back, deserves a call — even if the child looks okay.
Fever = 100.4°F / 38°C+
Warning signs
Keep it on your phone
The whole guide, pocket-sized. Save it to your phone and send it to your partner, your parents, whoever's watching the baby tonight.
Parent-friendly places to go deeper.
Questions
Have a question about this topic? I read every one and answer as many as I can, sometimes right here, sometimes in a video.
Questions and answers are for general education only and do not create a doctor-patient relationship. For medical advice specific to your child, always consult your pediatrician.
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