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By the Peanutbean parent · checked against CDC, NHS and AAP guidance · how we fact-check

Baby fever: when to worry, and what the number really means

Last verified: July 9, 2026

The short answer

  • A fever is 38C (100.4F) or higher
  • Most settle in 1 to 4 days as the body fights an infection
  • Your baby's age and how they look matter more than the number
  • Under 3 months, any fever means call straight away
Illustration of a thermometer

At every age, a baby who looks very unwell matters more than a high reading on the thermometer. This guide gives you the age rules and the warning signs, without the panic.

What counts as a fever

The NHS keeps it simple. A high temperature is 38C or more (NHS: High temperature in children). The AAP uses the same line in Fahrenheit: a temperature over 100.4F is a fever (AAP: Fever without fear).

38C = 100.4F the temperature that counts as a fever in a baby NHS and AAP

To check, use a digital thermometer. The NHS method for young children is the armpit: put the thermometer inside the top of the armpit, close the arm gently over it, hold it against the body, and wait as long as the instructions say. One caution: if your baby has just had a bath or been wrapped up warm, the reading can be high for a few minutes, so wait and try again.

A fever is not an illness. It is the body’s response to one. When the body detects an infection, the brain turns the temperature up to help fight it. The NHS calls a high temperature a natural and healthy response to infection. That framing is the whole point of this guide.

The number is not the emergency

This is the part most fever charts skip, and the AAP says it directly: the height of a fever does not tell you how serious the illness is. A child can have a high number and be basically fine once they are comfortable, and a child with a lower number can be the one who needs a doctor.

So the instinct to treat the thermometer is the wrong instinct. The AAP says not every fever needs treating, and the goal of any medicine is comfort, not a particular number. Watch how your baby looks and behaves. That tells you far more than the reading does. With one exception, which comes next.

The age rules that override everything

Under a certain age, a baby’s immune system is too new to take chances with, so the number does trigger a call regardless of how well they seem.

Call 111 or your GP now
  • baby under 3 months with a temperature of 38C or higher
  • baby 3 to 6 months with a temperature of 39C or higher

The AAP matches this: call the doctor right away for a baby younger than 3 months, that is 12 weeks, with a temperature of 100.4F (38C) or higher. The AAP adds one that applies at any age: a fever going above 40C, which is 104F, repeatedly.

If your baby is in these age groups and has a fever, do not wait to see how it goes. Make the call.

What to do at home for an older baby

If your baby is past those age thresholds and otherwise looks well, you can usually care for them at home while the fever runs its course over 1 to 4 days.

Care at home
  • give plenty of fluids, and if you are breastfeeding, carry on as normal
  • watch for signs of dehydration
  • offer food if they want it
  • check on them regularly, including during the night
  • keep them at home
  • give paracetamol or ibuprofen only if they are distressed, following the packet for their age and weight

On medicines, the safety rules are strict, so they are worth reading slowly. Do not give aspirin to a child under 16. Do not give paracetamol to a baby under 2 months. Do not give ibuprofen to a baby under 3 months, or under 5kg, or one who has chickenpox, is dehydrated, or has asthma unless a doctor has said it is fine. And do not alternate paracetamol and ibuprofen unless a health professional tells you to.

Just as important is what not to do. The NHS says do not undress your baby or sponge them down to cool them, because a fever is a healthy response, and do not pile on extra clothes or bedding. The AAP goes further and says cold or alcohol baths and ice packs are no longer recommended and can actually harm your child. One layer of clothing is right. If they are shivering, a blanket is fine.

Warning signs: call now

Some signs mean a fever needs a doctor’s eyes soon, whatever the number.

Call 111 or your GP now

The NHS says call if your child:

  • has a rash, as well as a high temperature
  • has a high temperature that has lasted 5 days or more
  • does not want to eat, or is not their usual self and you are worried
  • is dehydrated, with nappies that are not very wet, sunken eyes, or no tears when crying

Warning signs: call 999

And some signs are emergencies.

Call 999 now

The NHS says call 999 if your child:

  • has a stiff neck
  • has a rash that does not fade when you press a glass against it
  • is bothered by light
  • has a fit, a febrile seizure, for the first time
  • has unusually cold hands and feet
  • has skin that turns blue, grey, pale or blotchy, or blue or grey lips or tongue
  • is drowsy and hard to wake
  • will not stop crying, or seems confused
  • has difficulty breathing, which may look like grunting or the stomach sucking in under the ribs
  • is not responding normally or has no interest in feeding

A first febrile seizure is frightening to watch. It happens when a fever comes on, and while most are brief and harmless, the NHS says to call 999 the first time it happens, even if your baby seems to recover.

Above all, the NHS gives one instruction that sits over the whole list: trust your instincts. You know what your baby is normally like, so you will know when something is wrong. If you are worried and cannot say exactly why, that worry is reason enough to get help.

Keep reading

Sources

Peanutbean provides general information for educational purposes only. It is not medical advice. Every baby develops at their own pace. Always talk to your pediatrician about your child’s health.

Common questions

What temperature is a fever in a baby?

A high temperature is 38C, or 100.4F, or higher. Most fevers are the body fighting a common infection and settle within 1 to 4 days. Your baby's age and how they look matter more than the exact number.

Does a higher temperature mean a more serious illness?

No. The AAP is clear that the height of a fever does not tell you how serious the illness is. A baby who looks well with a high number can be less worrying than a baby who looks very unwell with a lower one. Watch the child, not just the thermometer, and follow the age rules.

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Peanutbean provides general information for educational purposes only. It is not medical advice. Every baby develops at their own pace. Always talk to your pediatrician about your child's health.