Fever in Babies: What the Thresholds Are and When to Call

The short answer

A fever in a baby means a body temperature of 100.4°F (38°C) or higher. Age decides how urgent that number is. The American Academy of Pediatrics says that if your baby is 3 months old or under and has a rectal reading at or above 100.4°F (38°C), contact your pediatrician right away, even without other symptoms. The NHS says call 111 under 3 months at 38°C or higher, and NICE places that group in a high-risk group for serious illness. In a baby that young, fever is not a symptom to watch. It is a reason to make contact now. The AAP, NHS and NICE each name a number for 3 to 6 months too. NICE stops relying on the reading alone only after 6 months, and the warning signs below apply whatever the thermometer says.

When to get help now

Call your emergency number (999 in the UK, 911 in the US, 000 in Australia) or go to an emergency department for any of the following. The NHS puts these in a red box headed "Immediate action required: Call 999 if your child:". It lists ten bullets there. All ten are below, in the NHS order, with nothing left out.

• Your baby has a stiff neck.
• Your baby has a rash that does not fade when you press a glass against it.
• Your baby is bothered by light.
• Your baby has a fit (febrile seizure) for the first time (they cannot stop shaking).
• Your baby has unusually cold hands and feet.
• Your baby has changes to their skin colour such as blue, grey, paler than usual or blotchy skin (this may be harder to see on brown or black skin), or blue, grey or paler than usual lips or tongue.
• Your baby is drowsy and hard to wake.
• Your baby is extremely agitated (does not stop crying) or is confused.
• Your baby has difficulty breathing (you may notice grunting noises or their stomach sucking under their ribcage), breathlessness or breathing very fast.
• Your baby is not responding like they normally do, or is not interested in feeding or normal activities.

Healthdirect Australia names a shorter set: call triple zero (000) for an ambulance if your child is drowsy, is having difficulty breathing, or has a stiff neck. Healthdirect also says take a baby under 3 months old with a fever above 38°C to the nearest hospital emergency department immediately, so for that age group its routing is emergency care, not a phone call.

Call for advice immediately

This combines all six items the NHS lists under "Urgent advice: Call 111 if your child:" with all eight items the AAP lists under "Call your child's doctor right away if your child has a fever and:", plus the AAP's 3-to-6-month and over-6-month thresholds from its separate "Fever and Your Baby" page, and the Healthdirect and NICE instructions. Each item names its source. Those sources say immediately or right away, and none gives a deadline in minutes.

• Your baby is under 3 months old and has a temperature of 38°C or higher, or you think they have a high temperature (NHS). The AAP puts that age group at 100.4°F (38.0°C) or higher and says call right away even without other symptoms. In Australia, see the Healthdirect emergency department instruction above.
• Your baby is 3 to 6 months old and has a temperature of 39°C or higher, or you think they have a high temperature (NHS). The AAP says notify your doctor at 101°F (38.3°C) or higher in this age range.
• Your baby is older than 6 months and has a temperature of 103°F (39.4°C) or greater (AAP).
• Your baby's temperature rises above 104°F (40°C) repeatedly, at any age (AAP).
• Your baby has other signs of illness, such as a rash, as well as a high temperature (NHS).
• Your baby does not want to eat, or is not their usual self and you are worried (NHS).
• Your baby is dehydrated, such as nappies that are not very wet, sunken eyes, and no tears when they are crying (NHS).
• The high temperature has lasted for 5 days or more (NHS).
• Your baby looks very ill, is unusually drowsy, or is very fussy (AAP).
• Your baby has other symptoms, such as a stiff neck, severe headache, severe sore throat, severe ear pain, breathing difficulty, an unexplained rash, or repeated vomiting or diarrhea (AAP).
• Your baby has had a seizure (AAP).
• Your baby has been in a very hot place, such as an overheated car (AAP).
• Your baby has immune system problems, such as sickle cell disease or cancer, or is taking steroids or other medicines that could affect their immune system (AAP).
• Your baby has heart problems that may affect how they tolerate a fever and the increased heart rate that results from it (AAP).
• Your baby seems less well than when you previously sought advice, or you are more worried than when you previously sought advice. NICE names both as reasons to seek further advice.

The AAP keeps a second, calmer heading, "Also call your child's doctor if:", with four items under it. All four are these: your child still "acts sick" once their fever is brought down, your child seems to be getting worse, the fever persists for more than 24 hours in a child younger than 2 years, and the fever persists for more than 3 days (72 hours) in a child 2 years of age or older. Those are reasons to call, and they are not on the right-away list above.

Two of the AAP right-away items name signs the NHS puts in its 999 box instead: a stiff neck and breathing difficulty. Where two bodies put the same sign in different tiers, act on the more urgent one. A stiff neck or difficulty breathing in a baby is an emergency call, not an advice call.

If you are worried about your baby right now, you do not need a reason that satisfies anyone. Call your pediatrician, your local out-of-hours service, or your emergency number. Trusting your instinct is the correct response.

What counts as a fever

HealthyChildren.org, the AAP's parent-facing site, states that a normal range is between 97.5°F (36.4°C) and 99.5°F (37.5°C), and that most pediatricians consider a temperature above 100.4°F (38°C) a sign of fever. The NHS says 38°C or more, and Healthdirect Australia 38°C or higher. Those three agree. That is three bodies, not a survey of every country.

Why age changes everything

Under 3 months, a fever is a reason for assessment on its own, regardless of how settled the baby seems. The Canadian Paediatric Society's position statement covers infants aged 90 days or younger presenting with a rectal temperature of 38.0°C or higher who appear well otherwise, and says ill-appearing infants should be assessed immediately.

The AAP also has a clinical practice guideline for well-appearing febrile infants 8 to 60 days old, built around a fever at or above 100.4°F (38°C). Its two further-management key action statements are hospitalization versus home and cessation of treatment, so the guideline expressly contemplates home as one outcome, decided by the clinician who examines the baby. The Canadian Paediatric Society adds that even well-appearing febrile young infants can deteriorate relatively rapidly. None of that is a prediction about your baby.

NICE is explicit that in children older than 6 months you do not use the height of body temperature alone to identify those with serious illness. A modest number in a baby who is limp, grunting or hard to rouse is not reassuring. The behaviour is the signal.

Where the guidance genuinely differs

Do not let anyone flatten these into one rule. The thresholds after 3 months differ. The AAP says notify your doctor at 101°F (38.3°C) or higher between 3 and 6 months, or 103°F (39.4°C) over 6 months. The NHS uses 39°C at 3 to 6 months as its call-111 trigger. NICE says to recognise that children aged 3 to 6 months with a temperature of 39°C or higher are in at least an intermediate-risk group for serious illness.

On measurement the difference is larger. HealthyChildren.org says a rectal reading is the most reliable and accurate for infants, especially babies under 3 months old, and is more accurate than taking it under the arm. NICE recommends an electronic thermometer in the axilla for infants under 4 weeks, and for children aged 4 weeks to 5 years an electronic or chemical dot thermometer in the axilla or an infra-red tympanic thermometer. NICE does not recommend the oral and rectal routes for this age group.

Both positions come from bodies that reviewed the evidence. On method, in the UK follow NHS and NICE, in the US follow the AAP, and in Australia follow Healthdirect. That is about how to take the temperature, not about whether to call. The emergency signs at the top of this page apply wherever you are. Your own clinician's instruction for your own baby comes first. Whichever method you use, tell whoever you call which method it was and what it read.

On devices, HealthyChildren.org's "Fever and Your Baby" page says to avoid using an ear thermometer for babies under 6 months old, since their ear canals are too small to allow an accurate reading. Its "Best Ways to Take a Temperature" page says ear thermometers are another acceptable choice whose accuracy depends on the beam reaching the eardrum, and that the AAP no longer recommends mercury thermometers because the glass may break and the vaporized mercury can be inhaled. NICE does not recommend forehead chemical thermometers.

If you have no thermometer, you are not stuck. NICE states that reported parental perception of a fever should be considered valid and taken seriously by healthcare professionals. The NHS 111 criteria include the phrase "or you think they have a high temperature". Say what you observed.

What a fever usually does, and where that stops being the point

The NHS says the temperature usually returns to normal within 1 to 4 days. That is the ordinary course. It is not a prediction about the baby in front of you, and it does not apply under 3 months, where the AAP, NHS, NICE and Healthdirect all say to make contact regardless.

The ordinary course stops being the story at any emergency sign listed at the top, any call-now situation, a fever lasting 5 days or more, a baby going downhill rather than levelling off, or your own sense that something is wrong. Any one of those is a call, not a wait.

On medicine

This page names no medicine and no dose, deliberately. Infant fever medicine is dosed by weight and by age, and restrictions differ between products and between countries. Ask your pediatrician or pharmacist what, if anything, is appropriate for your baby, and how much. Do not use a dose worked out for an older child, a different product strength, or a different baby.

The NHS says not to undress your child or sponge them down to cool them, because a high temperature is a natural and healthy response to infection, and not to cover them up in too many clothes or bedclothes.

Sleep while your baby is unwell

Illness does not change safe sleep. The AAP says babies who sleep on their backs are much less likely to die suddenly and unexpectedly than babies who sleep on their stomachs or sides, so put your baby on their back. Place them in a crib, bassinet, portable crib or play yard that meets the safety standards of the Consumer Product Safety Commission, on a firm surface. The AAP gives that definition in two sentences, and the second one matters most on a page about an unwell baby: "A firm surface means that it shouldn't indent when your baby is lying on it. Any surface that inclines more than 10 degrees isn't safe for your baby to sleep on." A congested or feverish baby is not an exception to that. An inclined seat, a swing, a car seat out of the car, or a propped mattress is not a sleep surface. Keep soft objects and loose bedding out, because the AAP says these can increase your baby's risk of entrapment, suffocation or strangulation. The AAP recommends room sharing for at least the first 6 months. On sharing a bed, the AAP's own wording is that based on the evidence, the AAP doesn't recommend bed sharing with your baby under any circumstances. A feverish baby is one you may want closer, and room sharing is how to do that.

What this page is

This page explains general published guidance from named bodies. It cannot tell you anything about your baby. History, gestational age at birth, existing conditions and medications all change how a fever is assessed, and that assessment belongs to the clinician who knows them. Nothing here replaces it, and nothing here should slow down your call.

Related baby-care guides

Continue with dehydration signs in babies and newborn what is normal first two weeks.

Sources

• Fever and Your Baby, HealthyChildren.org (AAP). Rectal definition, under-3-months instruction, age thresholds, ear thermometers under 6 months.
• Fever: When to Call the Pediatrician, HealthyChildren.org. All eight items under the AAP's "Call your child's doctor right away" heading, including the 104°F (40°C) item, and all four items under its separate "Also call your child's doctor if:" heading.
• Signs and Symptoms of Fever, HealthyChildren.org. Normal range.
• Best Ways to Take a Temperature, HealthyChildren.org. Rectal versus under-the-arm, ear thermometers, mercury. States no age limit for ear thermometers.
• High temperature (fever) in children, NHS, reviewed 3 January 2024. The 38°C definition, 1 to 4 days, all ten 999 bullets and all six 111 items above.
• Fever in under 5s, NICE guideline NG143, full text via NCBI Bookshelf. Risk groups, the over-6-months rule, measurement, parental perception, safety netting. The nice.org.uk page returned 403 to this writer, so the mirror is cited.
• Infant Fever Clinical Practice Guideline, AAP. The 8 to 60 day scope, age bands, key action statements.
• Well-appearing febrile young infants aged 90 days or younger, Canadian Paediatric Society. The 38.0°C scope, ill-appearing infants, deterioration.
• Fever in children, Healthdirect Australia. The 38°C definition, emergency department instruction, triple zero triggers.
• A Parent's Guide to Safe Sleep, HealthyChildren.org. Back sleeping, firm surface, CPSC sleep space, bedding, room sharing, bed sharing.

Independent guidance, not medical advice. We do not diagnose or recommend treatments or doses. For any concern about your baby, contact your pediatrician or local emergency service.