When to call

Call an ambulance or go to the emergency department now

Temperature of 38C (100.4F) or higher in a baby under 3 months, or a temperature below 36C (96.8F) at any age.
Your baby stops breathing, has pauses in breathing, or is struggling to breathe.
Grunting with each breath, breathing over 60 breaths a minute, or chest indrawing.
Skin, lips or tongue that look pale, ashen, mottled, blue or grey. On brown and black skin, check the palms, soles, lips, gums and inside the eyelids.
A non-blanching rash — spots that look like small bruises and do not fade when you press a glass against them.
Unresponsive or unrousable, limp or floppy, or not responding as they normally do.
A weak, abnormally high-pitched or continuous cry.
A bulging fontanelle (the soft spot on the head) or neck stiffness.
Seizures, or stiffness, shaking or jerking.
Bilious vomiting — green or yellow-green vomit — or frequent forceful (projectile) vomiting.
Diarrhoea with dehydration.
Your baby is under 8 weeks old and you are very worried about them.
A severe allergic reaction, or a first febrile seizure even if they seem to recover.

Source: NICE NG194 red flags for serious illness in babies; NICE NG143; NHS

Everything above is taken from published guidance, not from a general sense of what sounds bad. The list of red flags is NICE's own, from recommendation 1.4.9 of its postnatal care guideline (NG194), supplemented by the NHS's advice on when to call an ambulance for a baby.

The fever threshold, and why it is so low

This is the single most important number for a newborn.

NICE's guideline on fever in under 5s (NG143) states: "Recognise that children younger than 3 months with a temperature of 38°C or higher are in a high-risk group for serious illness." A temperature of 38C or over in a baby under 3 months sits in the red column of NICE's traffic light system — the highest-risk category.

The AAP sets the identical threshold from the other side of the Atlantic. Call your child's doctor right away if your child "is younger than 3 months (12 weeks) and has a temperature of 100.4°F (38.0°C) or higher."

For slightly older babies, NICE's next threshold is: "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." That is 102.2F.

NICE also notes that some vaccinations can induce fever in children under 3 months — but that is a judgement for a clinician to make, not a reason to wait at home.

The catch: a normal temperature is not reassurance

NICE's postnatal care guideline says something that should change how you use a thermometer: "Be aware that fever may not be present in young babies with a serious infection." And elsewhere: "Be aware that the presence or absence of individual symptoms or signs may be of limited value in identifying or ruling out serious illness in a young baby."

The NHS says the same in parent-facing language, and adds a specific pattern to watch for: "your child is quiet and listless, even when their temperature is not high." A high temperature with cold feet and hands is another combination the NHS lists as concerning.

What NICE counts as red

NICE's traffic light system for fever in under 5s gives a full red column. In babies, the ones most relevant are: pale, mottled, ashen or blue skin, lips or tongue; no response to social cues; appearing ill to a healthcare professional; not waking, or if roused not staying awake; a weak, high-pitched or continuous cry; grunting; a respiratory rate greater than 60 breaths per minute; moderate or severe chest indrawing; reduced skin turgor; a bulging fontanelle; a non-blanching rash; neck stiffness; and focal neurological signs or focal seizures.

NICE's instruction to clinicians for children with red features who are not immediately life-threatening is telling: they "should be urgently assessed by a healthcare professional in a face-to-face setting within 2 hours."

One further NICE warning, which is why neck stiffness is not the test people think it is: "the classic signs of meningitis (neck stiffness, bulging fontanelle, high-pitched cry) are often absent in infants with bacterial meningitis."

The amber signs — see someone, but not by ambulance

Ask for an urgent GP appointment or ring your out-of-hours service if your baby:

Is aged 3 to 6 months with a temperature of 39C (102.2F) or higher.
Has a high temperature that does not come down with medicine — and note the age limits below before you give any.
Is feeding much less than normal, or not feeding normally and you are worried.
Has nappies that are drier than usual, or reduced urine output — a sign of dehydration.
Has rapid breathing or panting, or a throaty noise while breathing.
Is crying constantly and cannot be consoled or distracted, or the cry does not sound like their normal cry.
Is hard to wake up, or appears disoriented or confused.
Has a capillary refill time of 3 seconds or longer — NICE classes this as an amber sign.
Has had a fever lasting 5 days or longer. NICE says these children should be assessed for Kawasaki disease.

The NHS gives useful routing advice: during the day Monday to Friday, call your GP surgery; evenings and weekends, call NHS 111. And a practical point specific to small babies: "if your baby is under 6 months old it's hard for a doctor or nurse to assess them over the phone – you can go to an urgent treatment centre or, if you're very worried, take them to A&E."

Medicines: the age limits that catch people out

The NHS gives two hard limits for babies:

Do not give paracetamol to a baby under 2 months.
Do not give ibuprofen to a baby under 3 months or under 5 kg (11 lb), unless prescribed by a doctor.

NICE adds a broader point about fever medicine: "Do not use antipyretic agents with the sole aim of reducing body temperature in children with fever", and "when a child has been given antipyretics, do not rely on a decrease or lack of decrease in temperature at 1 to 2 hours to differentiate between serious and non-serious illness." A fever that comes down with paracetamol does not rule anything out.

The AAP adds one more threshold at any age: call if the temperature "rises above 104°F (40°C) repeatedly."

The newborn-specific things worth a call

The AAP's list of what to ring about in the first weeks, beyond the red flags:

Any signs of jaundice — yellowing of the skin or the whites of the eyes.
Hardness in your baby's abdomen, especially if they have not pooed in the last 24 hours.
Frequent coughing or gagging while feeding.
Excessive crying, especially if you sense pain.
Failure to wake up for feeds, or disinterest in eating. The AAP notes that lethargy — "especially if it's a sudden change in her usual pattern" — may be a symptom of serious illness.
Blood in a bowel movement, which usually means a small anal tear but should be confirmed.

Trust your instinct — that is the guidance, not a platitude

NICE's first recommendation on illness in babies is directed at clinicians and is worth knowing: "Listen carefully to parents' concerns about their baby's health and treat their concerns as an important indicator of possible serious illness in their baby." It also tells them to be alert to "a change in the baby's behaviour or signs, such as refusing feeds or a change in the level of responsiveness."

The NHS says the same to parents: "You know better than anyone else what your child is usually like, so you'll know when something is seriously wrong. Trust your instincts."

If you are reading this at 3am wondering whether you are overreacting: the guidance says your concern is data. Ring.

When to Call a Doctor About a Newborn