Fever in a baby under 3 months: why it is treated as urgent

Most fevers in older children are the body's natural response to a cold, a throat infection or a vaccination, and the NHS says a high temperature usually returns to normal within 1 to 4 days. In very young babies the picture changes. NICE places children younger than 3 months with a temperature of 38C or higher in a high-risk group for serious illness, and the NHS tells parents to call 111 for a baby of that age with 38C or higher, or if they think the baby has a high temperature.

NICE's guideline is written for health professionals. For this age group it sets out a defined set of observations and tests, described below, rather than a watch-and-wait approach.

The NHS recommends a digital thermometer placed in the top of the armpit, with the arm closed gently over it and held to the side, left for as long as the instructions say. If the baby has just had a bath or been wrapped in a blanket, wait a few minutes and try again. NICE says that in infants under 4 weeks, healthcare professionals should measure temperature with an electronic thermometer in the armpit, and that forehead chemical thermometers are unreliable. It also says that a parent's perception of fever should be taken seriously.

The NHS says a child with a high temperature might feel hotter than usual when you touch their back or chest, feel sweaty or look unwell, and for a baby under 3 months it says to call 111 if you think they have a high temperature, even without a confirmed reading.

The NHS says to call 999 if a child with a high temperature:

• has a stiff neck, is bothered by light, or has a rash that does not fade under a glass

• has a fit for the first time and cannot stop shaking

• has unusually cold hands and feet, or blue, grey, pale or blotchy skin, lips or tongue

• is drowsy and hard to wake, extremely agitated, confused or not responding as normal

• has difficulty breathing, such as grunting, the stomach sucking under the ribs, or very fast breathing

• is not interested in feeding or normal activities

NICE sets out the approach a paediatric team follows for infants under 3 months with fever. Their heart rate, breathing rate and temperature are observed and recorded, and tests include a full blood count, blood culture, C-reactive protein and a urine test for urinary tract infection, with a chest X-ray only if breathing signs are present and a stool culture if there is diarrhoea.

NICE says a lumbar puncture should be performed, unless contraindicated, in infants younger than 1 month and in infants aged 1 to 3 months who appear unwell or have a particular white cell count pattern. It also says antibiotics given by injection or drip should be given to infants younger than 1 month with fever and to those aged 1 to 3 months who appear unwell. These are decisions for the treating team, not something parents can judge at home.

The NHS advice on medicines depends on age. It says not to give paracetamol to a child under 2 months and not to give ibuprofen to a child under 3 months, weighing under 5kg, with chickenpox or dehydrated. It also says not to undress a child or sponge them down to cool them, and not to cover them in too many clothes or bedclothes. For a breastfed baby, the NHS says to continue to breastfeed as normal and to look out for signs of dehydration.

Because the advice for babies this young is to seek assessment first, home treatment is not a substitute for being seen.

NHS inform adds a point that surprises many parents: a temperature of 36C or below in a baby under 3 months is also a reason to call 999 or go to A&E, if the baby has not had a vaccination in the past 2 days. It also lists a weak, high-pitched cry that is not their normal cry. NICE notes that classic signs of meningitis, such as neck stiffness, a bulging fontanelle or high-pitched cry, are often absent in infants with bacterial meningitis, which is a further reason why a baby who seems unwell should be seen even without these signs.

The NHS lists vaccinations among the causes of a high temperature in children, and NICE and NHS inform both note the vaccination effect in babies under 3 months. The NHS call-111 advice for a baby under 3 months does not mention a vaccination exception, so the cautious course is to ring 111, say when the vaccine was given and let a clinician interpret the timing.