Baby temperature checker

Why the same number means different things

A temperature of 38.2°C in a two-month-old and the same reading in a two-year-old are not the same event. The first sits in the red column of NICE's traffic light table, alongside a non-blanching rash and a bulging fontanelle. The second sits in no column at all.

That is not because a fever is more dangerous in a newborn as such. It is because very young babies show few localising signs, can become unwell quickly, and give a clinician much less to go on. The threshold is lower precisely because the information is thinner.

Under three months is the line that matters

NICE puts "age less than 3 months, temperature 38°C or above" in the red, high-risk column. NHS inform goes further at the other end and lists a temperature of 36°C or below in an under-three-month-old, with no vaccination in the previous two days, as a reason to phone 999.

For a baby this young, waiting to see how things develop is not the recommended approach. Ring 111 for advice, or 999 if any of the emergency signs are present. Most such babies turn out to have something ordinary. The point of the low threshold is that the small proportion who do not are hard to pick out early by eye.

Three to six months, and then everyone else

Between three and six months the threshold rises to 39°C and the colour changes from red to amber — intermediate risk, meaning a call today rather than an emergency. Over six months, NHS inform still lists 39°C or above as a reason to contact the GP practice, but the balance of the assessment has shifted decisively away from the thermometer.

Beyond the newborn period, how high the temperature is turns out to be a poor guide to how ill a child is. A child at 39.5°C who is drinking, playing and responding to you is usually less worrying than a child at 38.2°C who is floppy and refusing feeds. NHS inform says this outright: if your child seems otherwise well, it is less likely they are seriously ill.

The signs that outrank the number

The traffic light table exists because a temperature on its own tells a clinician relatively little. What sits alongside it does the work: colour, activity, breathing, and circulation.

The red features include pale, mottled, ashen or blue skin, lips or tongue; no response to social cues; a weak, high-pitched or continuous cry; not waking or not staying awake when roused; grunting; a respiratory rate over 60 breaths a minute; moderate or severe chest indrawing; a non-blanching rash; a bulging fontanelle; neck stiffness; and reduced skin turgor.

The amber features include pallor reported by a parent, not responding normally to social cues, no smile, waking only with prolonged stimulation, decreased activity, nasal flaring, dry mucous membranes, poor feeding in infants, reduced urine output, and a capillary refill time of three seconds or more.

Any of those, at any temperature, matters more than the reading. And NICE adds a line worth quoting to anyone who tells you that you are fussing: reported parental perception of a fever should be considered valid and taken seriously.

How you measure changes what you get

NICE gives explicit instructions on route, and they are narrower than the shelf in a chemist suggests. For infants under four weeks, use an electronic thermometer in the armpit. From four weeks to five years, use an electronic or chemical dot thermometer in the armpit, or an infra-red tympanic thermometer in the ear.

Oral and rectal routes are not recommended in this age group. And forehead chemical thermometers — the plastic strips you hold against the skin — are described flatly as unreliable and not to be used by healthcare professionals. If that is what produced your reading, take it again with something else before acting on the number. If your baby looks unwell, act on how they look rather than waiting for a better reading.

Celsius, Fahrenheit, and why both numbers follow you around

UK thermometers and UK guidance work in Celsius. A great deal of what a search engine returns, including most American parenting advice, works in Fahrenheit. The two most quoted "fever" numbers, 38°C and 100.4°F, are the same temperature.

The conversion is simply degrees Celsius times nine-fifths plus thirty-two. It is worth doing carefully in one specific situation: a thermometer bought abroad, or one whose display mode has been changed by a curious toddler, can produce a reading that looks alarming or reassuring for entirely the wrong reason. A 37° reading is normal; a 37°F reading would be a medical emergency of a completely different kind. If a number looks impossible, check the unit before you check anything else.

Fever after vaccinations

A raised temperature in the day or two after immunisations is common and expected. That is why the NHS advice for very young babies carries the qualifying phrase "and has not had a vaccination in the past 2 days". If your baby was vaccinated in the last 48 hours, say so when you call — it changes how the reading is interpreted.

What this page will not give you

No medicine names and no amounts appear anywhere on it, deliberately. Fever medicines for children are measured out by weight and age, the right amount for your child is printed on the bottle you actually own, and any pharmacist will confirm it in under a minute at no cost. NICE's own position is also worth knowing: antipyretics are not always needed, and if a child is not distressed by the fever there is no requirement to lower the number for its own sake.