When to Go to the Emergency Room for a Fever in a Baby

### Under three months old: a fever is not something to sleep on The American Academy of Pediatrics is unambiguous. If your baby is three months of age or younger and has a rectal temperature of 100.4°F (38°C) or higher, call your pediatrician immediately — at any hour, and even if the baby otherwise seems well. If it is the middle of the night, or you cannot reach anyone, go to an emergency room. Both of ours are open every hour of every day.

The younger the baby, the lower the number that matters — and under three months, any fever at all is the threshold. A very young baby cannot contain an infection the way an older child can, so a temperature may be the only outward sign that something significant is happening.

The age bands, as the AAP gives them

Age	Contact your doctor at
3 months or younger	100.4°F (38°C) or higher — call immediately
3 to 6 months	101°F (38.3°C) or greater
Older than 6 months	103°F (39.4°C) or higher

Those are the American Academy of Pediatrics’ own figures, and they are the ones a paediatrician would act on. Only the first carries the word immediately.

These are the numbers at which to make contact. They are not a permission structure. A temperature below the line for your baby’s age does not mean a problem is not worth a call, and nothing on this page is a reason to wait when you are worried.

Which thermometer, and at what age

This part changes the number you are reading, so it is not a detail.

Use a rectal thermometer for infants. The AAP describes it as the most reliable and accurate for babies, especially those under three months old — which is precisely the age where the threshold is lowest and the decision most consequential.

Do not use an ear thermometer on a baby under six months. The AAP’s reason is anatomical rather than a matter of preference: their ear canals are too small to allow an accurate reading.

A forehead or ear reading is fine for noticing that something has changed in an older child. It is not what you want to be making a 2am decision on for a newborn.

What we pay attention to, beyond the number

Speaking for ourselves rather than citing anyone: we pay more attention to how a baby looks and behaves than to the figure on the thermometer. A settled baby who is feeding and recognisable as themselves at a higher number worries us less than a floppy, poorly feeding, inconsolable baby at a lower one.

That is a statement about what we act on, and it works in one direction only — it is a reason to come in when the number looks reassuring and the baby does not. It is never a reason to stay home because the number seems low.

Regardless of the temperature, these are worth a call: poor feeding or refusing to feed, unusual sleepiness or difficulty waking, persistent irritability or inconsolable crying, fewer wet nappies than usual, or simply a baby who does not seem like themselves to the person who knows them best.

Call 911 for these

The National Library of Medicine’s guidance on recognising medical emergencies names these in a child as reasons to call for emergency help rather than to make an appointment:

• Skin that looks blue
• A fever with a stiff neck
• Bleeding that will not stop
• A child who cannot stand

To which add the ones that are emergencies in anyone: difficulty breathing, a seizure, unresponsiveness, or a rash of small dark red or purple spots that do not fade when you press on them.

And anything where you genuinely believe your child could be in danger. You do not need to be right.

Where to take a baby with a fever

During clinic hours, your paediatric practice is the right door, because they have the history — what the growth curve looks like, what the last infection was, what is already going round. Mercy Grace Pediatrics & Primary Care in Gilbert is built for children specifically.

Outside those hours, or for anything on the 911 list, or for any fever in a baby under three months, an emergency room. Both of ours are open every hour of every day, walk-in, with a full laboratory in the building — which matters here, because in a young baby with a fever the useful next step is usually testing rather than reassurance.

We are not a pediatric emergency room, and we treat children every day. Both halves of that matter. Our emergency rooms are equipped and staffed to assess and stabilise a child, and where a child needs a specialist level of paediatric care, we arrange the transfer rather than keep them.

About medication

This page gives no doses, deliberately. Fever medicine for a child is dosed by weight rather than by age, the right figure depends on the specific product in your cupboard, and a number printed on a web page is wrong for some child reading it.

If you are unsure of a dose, call and ask — we would far rather answer that than have you estimate. Out of hours, a pharmacist can check a weight-based dose with you. And if you think a child has had too much of anything, Poison Control answers at any hour on 1-800-222-1222.

Questions people actually ask

The fever came down with medicine. Does that change anything? Not for a baby under three months. And in an older child, a fever that responds to medicine tells you about the medicine, not about the cause.

She has a fever but she is playing happily. In a baby under three months, the number is still the threshold and the call is still immediate. In an older child, how they look genuinely matters — and it is a reason to come in when they look wrong, never a reason to stay home when you are worried.

Is teething causing this? Teething is not a reliable explanation for a significant fever, and treating it as one is how a real infection gets a day’s head start.

How do I take a rectal temperature? Your paediatric practice will walk you through it on the phone, and it is worth asking before you need to know at 2am.

I feel like I am overreacting. “I am probably overreacting” is a sentence we hear constantly and it is almost never true. Bring them in.

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For a fever in a baby under three months, or anything on the 911 list above, do not wait. Both of our emergency rooms are open 24 hours a day, every day of the year — walk in, no appointment and no referral, with a full laboratory in the building. Find your nearest location →

Related reading: which door do you need · primary care and pediatrics across our network

Sources

3. Recognizing medical emergencies — MedlinePlus Medical Encyclopedia, U.S. National Library of Medicine. https://medlineplus.gov/ency/article/001927.htm
9. Fever and Your Baby — American Academy of Pediatrics, HealthyChildren.org. https://www.healthychildren.org/English/health-issues/conditions/fever/Pages/Fever-and-Your-Baby.aspx

This article is general information, not a diagnosis, and it cannot account for your child’s history or medications. The age bands above are the American Academy of Pediatrics’ figures; other authorities publish slightly different numbers for the older bands, which is one more reason that a call is always reasonable. If you are unsure, come in — this list is not exhaustive and you are not expected to diagnose your child. If it might be an emergency, treat it as one and call 911.