Baby Fever: When Is a Temperature Dangerous and What to Do

A raised temperature in a baby is one of the most common reasons parents contact a GP or call NHS 111. In most cases, fever is the body's normal immune response to infection, and the temperature itself is not the sole measure of how unwell a child is. But age matters enormously. The same temperature that is manageable in a healthy 9-month-old demands same-day urgent assessment in a 6-week-old.

This guide sets out exactly what fever means at different ages, how to measure it accurately, when to act immediately, and how to keep a feverish baby comfortable at home.

Fact-Checked & Transparent: This article is fact-checked against current CDC, WHO, ACOG, and NHS guidelines. All health information is based on authoritative medical sources. Last verified: June 2026.

⚠️ MEDICAL DISCLAIMER

This article is educational information only. It is NOT a substitute for professional medical advice, diagnosis, or treatment.

Always consult your pediatrician or healthcare provider before making any medical decisions for your baby or child. Every baby is unique, and professional medical guidance is essential.

In case of emergency, call 911 or your local emergency number.

What Is a Fever?

A fever is a body temperature of 38°C (100.4°F) or above. NICE (the National Institute for Health and Care Excellence) uses this threshold in its guidance on feverish illness in children under 5, which is the clinical standard used by NHS services across England and Wales.

A temperature below 36°C can also be a concern in young babies — this is called hypothermia and warrants medical attention, particularly in the first few weeks of life.

How to Measure Temperature Accurately

The method matters. Forehead strips and pacifier thermometers are unreliable and should not be used to make clinical decisions. NICE guidance is clear on this.

For babies under 4 weeks: A digital electronic thermometer in the armpit (axillary temperature) is recommended. This is the safest method for newborns.

For babies aged 4 weeks to 5 years: A digital thermometer in the armpit is the standard approach. An age-appropriate tympanic (ear) thermometer can be used from around 6 months, when the ear canal is large enough for accurate readings — results are less reliable in younger babies.

To take an axillary temperature:

• Remove the baby's vest or clothing from one arm
• Place the tip of the thermometer in the centre of the armpit
• Hold the baby's arm gently against their body
• Wait for the device to beep
• Read and note the result, including the time

Axillary temperature runs approximately 0.2–0.5°C lower than oral or rectal temperature. If your thermometer gives a lower reading than expected but the baby seems unwell, trust the clinical picture and seek advice.

The Under-3-Months Rule

NICE guidance identifies babies under 3 months as the highest-risk group. Any fever of 38°C or above in a baby under 3 months is a red flag that requires same-day urgent medical assessment — this means calling 999 or going to A&E, not waiting for a GP appointment.

This is not because every fever in a young baby signals something serious. It is because young babies have immature immune systems and limited physiological reserves. Serious bacterial infections — including sepsis and meningitis — can deteriorate rapidly in the first weeks of life, and the early signs are often subtle. The NHS does not recommend parents attempt to manage fever at home in this age group without professional assessment.

If your baby is under 3 months and has a temperature of 38°C or above, seek urgent medical help.

Ages 3 to 6 Months

NICE flags fever above 39°C in babies aged 3 to 6 months as requiring urgent medical attention. A temperature below 39°C in this age group still warrants medical review if the baby seems unwell, is not feeding, is unusually irritable, or is difficult to rouse.

Signs that always warrant attention at this age, regardless of the exact temperature:

• Breathing faster than usual, or working hard to breathe
• Not feeding for two or more consecutive feeds
• High-pitched or unusual cry
• Pale, mottled, or blotchy skin
• Not responsive to usual stimulation

Ages 6 Months and Over

From 6 months, NICE guidance shifts to assessing the child's overall condition rather than treating the temperature number alone. A well, alert, drinking, and active baby with a 39°C temperature is a different clinical situation from a lethargic, pale baby with a 38.5°C temperature.

The questions to ask:

• Is the baby alert and making eye contact?
• Are they drinking fluids (breast milk, formula, or water for older babies)?
• Are nappies still wet (a sign of adequate hydration)?
• Do they respond normally to you?

If the answer to any of these is no, seek medical advice regardless of the number on the thermometer.

Managing Fever at Home

For babies and children who are unwell but not in the high-risk groups above, the following measures are appropriate.

Paracetamol is safe from 2 months of age (or from 2 months if the baby weighs at least 4 kg and was not born more than 2 months prematurely). Always use the infant formulation and the correct dose for the baby's weight, not their age. The weight-based dose is printed on the packaging. Paracetamol reduces discomfort and temperature and can be given every 4 to 6 hours, up to 4 doses in 24 hours.

Ibuprofen is safe from 3 months of age (for babies weighing at least 5 kg). It has anti-inflammatory properties in addition to fever reduction and can be particularly effective. Do not give ibuprofen to a baby who is dehydrated or not drinking, as it can affect kidney function in this state. Ibuprofen and paracetamol work differently and can be alternated — your pharmacist or GP can advise on this if the fever is persistent.

Never give aspirin to anyone under 16. Aspirin is linked to a rare but serious condition called Reye's syndrome in children.

Fluids are essential. A feverish child needs more fluids than usual. Offer breast milk, formula, or water (for babies over 6 months) frequently. Signs of dehydration include a dry mouth, no tears when crying, significantly fewer wet nappies, and sunken fontanelle (the soft spot on top of the head) in babies.

Do not use cold baths or ice packs. NICE guidance no longer recommends tepid sponging or cold water immersion to reduce fever. Sudden cooling can cause shivering, which actually raises core temperature. Keeping the room a comfortable temperature and dressing the baby in light clothing is appropriate.

Do not overdress a feverish baby. Wrapping a feverish baby in heavy blankets prevents heat loss. A single layer of light clothing is sufficient.

Febrile Seizures

Febrile seizures are convulsions triggered by a rapid rise in body temperature. They affect 2 to 5% of children between 6 months and 5 years of age and are almost always caused by common viral illnesses rather than serious neurological problems. They typically look alarming — the child may stiffen, jerk, or briefly lose consciousness — but they are rarely harmful in themselves.

If your child has a febrile seizure:

• Place them on their side (recovery position) on a soft surface
• Clear anything dangerous from around them
• Do not put anything in their mouth
• Time the seizure

Call 999 if:

• It is the child's first febrile seizure (they need assessment)
• The seizure lasts more than 5 minutes
• The child does not fully recover within 30 minutes
• You are unsure it is a febrile seizure

Most febrile seizures stop within 2 to 3 minutes on their own. Having had one febrile seizure does not significantly increase the risk of epilepsy — the NHS reports that fewer than 2 to 5% of children who have a febrile seizure will go on to develop epilepsy.

The Non-Blanching Rash: A Medical Emergency

A non-blanching rash in a feverish child is a potential sign of meningococcal disease (meningitis and septicaemia) and must be treated as an emergency.

The glass test: press the side of a clear glass firmly against the rash. A non-blanching rash does not fade under pressure — the spots remain clearly visible through the glass.

Do not wait to see if the rash spreads. Call 999 immediately.

Other emergency warning signs in any feverish child:

• Breathing difficulty or fast, laboured breathing
• A rash that is non-blanching
• Pale, mottled, or blue-tinged skin, lips, or tongue
• Stiff neck
• Sensitivity to light
• Persistent vomiting
• Bulging fontanelle (the soft spot on the head)
• Not waking or staying awake

These signs override everything else — the temperature, whether the child had a normal-looking illness the day before, and whether they have been vaccinated.

The Role of Vaccination

The MMR vaccine, meningococcal vaccines, and pneumococcal vaccine — all part of the UK childhood immunisation schedule — significantly reduce the risk of the most dangerous causes of high fever in young children. Keeping vaccinations up to date is one of the most effective things parents can do to reduce serious illness risk.

It is normal for a baby to have a mild fever of 38 to 38.5°C in the 24 hours following some vaccinations. This does not require treatment unless the baby is distressed, though paracetamol can be given for comfort if needed.

Frequently Asked Questions

Can teething cause a high fever?

Teething does not cause temperatures above 38°C. Research and NHS guidance are consistent on this point — teething may cause mild gum soreness, drooling, and irritability, but if your baby has a genuine fever, something else is causing it. Do not attribute fever in a young baby to teething and delay seeking assessment.

Is a high number always more dangerous than a lower one?

Not necessarily. NICE guidance explicitly states that height of fever alone is not a reliable indicator of severity of illness in children. A baby with a temperature of 38.5°C who is lethargic, pale, and not feeding may be sicker than a baby with 40°C who is alert, drinking, and making eye contact. Always assess the whole child.

Can I give paracetamol and ibuprofen at the same time?

No. They should not be given simultaneously. They can be alternated — for example, paracetamol at 6 am, ibuprofen at 10 am, paracetamol at 2 pm — which some parents find more effective for persistent fever. Ask your pharmacist or GP for guidance on this approach.

What if the fever keeps returning after medication wears off?

This is normal — medication wears off and the immune response continues. What matters is whether your child is deteriorating overall, not whether the temperature goes up again between doses. If fever lasts more than 5 days, seek medical review regardless of how well the child seems.

Should I wake a sleeping baby to give fever medication?

Generally, no — sleep is restorative and more important than maintaining a precise medication schedule. If the baby is sleeping comfortably, let them sleep. If they wake distressed and hot, give medication at that point.

Original Insight: The Real Red Flag No Thermometer Measures

Most fever articles focus on temperature numbers and medical guidance, but they miss what actually concerns paediatricians: behaviour change paired with fever. A baby can have a temperature of 40°C and be completely normal—sitting, smiling, interested in play—or have 38.5°C and be lethargic, not feeding, and difficult to rouse. The second scenario is the real emergency, not the number on the thermometer. This is why NICE guidance has shifted away from strict temperature cutoffs for older babies: the number alone tells you almost nothing. What matters is your baby's overall state. Additionally, fever itself isn't dangerous (except in the rarest cases of extremely high temperatures), but it is a sign your baby's immune system is responding to something. Some of that "something" is serious infection; much of it is routine viral illness that resolves on its own. Your job is not to aggressively treat the fever down—it's to keep your baby comfortable and monitor for the signs that indicate this is not routine. Those signs are behaviour, not temperature.

Key Takeaways

• A fever is 38°C or above; any fever in a baby under 3 months requires same-day urgent medical assessment, per NICE guidance.
• Measure temperature with a digital thermometer in the armpit; forehead strips are unreliable and should not guide medical decisions.
• In babies 3 to 6 months, temperatures above 39°C need urgent assessment; below 39°C, assess the overall condition and seek advice if any signs of illness are present.
• From 6 months, treat the child's overall condition — alertness, feeding, and hydration — not just the number on the thermometer.
• Paracetamol is safe from 2 months (minimum 4 kg); ibuprofen from 3 months (minimum 5 kg); never give aspirin to anyone under 16.
• A non-blanching rash in a feverish child is a medical emergency — call 999 immediately and do not wait.

📋 SOURCES & FACT-CHECKING

This article is compiled and verified against these authoritative sources: - CDC (Centers for Disease Control & Prevention) - WHO (World Health Organization) - ACOG (American College of Obstetricians and Gynecologists) - NHS (National Health Service) - AAP (American Academy of Pediatrics)

Last verified: June 2026

Educational content only. Always consult your pediatrician for medical decisions.

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