Fever in babies under 3 months: why it is always urgent
The one rule that never changes
If your baby is under 3 months old and has a temperature of 38°C (100.4°F) or above, call 111 or your GP straight away. Do not wait to see if it goes down on its own. Do not give paracetamol and monitor at home. Call now. This rule applies 24 hours a day, 7 days a week, regardless of how your baby looks or whether they seem otherwise well.
This is not meant to alarm you. The vast majority of newborns who are seen with a fever turn out to have a viral infection that clears without any lasting harm. But the reason doctors and the NHS treat any fever in this age group as urgent is well-founded, and understanding it can help you feel confident acting quickly if you ever need to.
Why under-3-month fever is different from older babies
A baby's immune system is not fully developed at birth. In the first weeks and months of life, the immune system is still learning to recognise and fight pathogens. Crucially, very young babies cannot localise an infection the way older children can. In an older baby or child, a bacterial infection might stay in the ear, throat, or chest. In a newborn, bacteria can enter the bloodstream and spread to the brain or other organs much more easily. This is called sepsis, and in newborns it can progress within hours.
Serious bacterial infections (SBIs) are more common in babies under 3 months than in older children. The pathogens most likely to cause problems in this age group include group B Streptococcus (GBS), Escherichia coli, Listeria, and Streptococcus pneumoniae. A newborn's fever may be the only early sign of a serious infection, and waiting for other symptoms to develop is not a safe strategy.
Why the threshold is lower than for older babies
For babies aged 3 to 6 months, the NHS uses a slightly higher threshold of 39°C as one of the amber warning signs. For babies over 6 months, fever alone is less immediately alarming if the baby is feeding and behaving normally. But for babies under 3 months, 38°C is the line because the risk of a serious bacterial infection is genuinely higher and the consequences of missing it are more severe. The lower threshold reflects caution, not excessive worry.
What to do while you wait for help
Once you have called 111 or your GP, keep your baby calm and close to you. If your baby is feeding, continue to offer feeds as you normally would. Skin-to-skin contact can help keep your baby settled. Watch your baby's breathing and look for any signs of increased difficulty, such as nostrils flaring, the chest or stomach pulling in sharply with each breath, or very rapid breathing. If your baby becomes very pale, mottled, or limp, or if breathing becomes laboured, call 999 immediately rather than waiting for a callback.
Do not give paracetamol or ibuprofen before you have spoken to a medical professional. Giving medication to reduce the fever can mask the temperature reading and make it harder for doctors to assess your baby when they are seen. Do not undress your baby to cool them down unless you are advised to do so.
What doctors will do: the full septic screen
When you bring a baby under 3 months in with a fever, doctors will carry out what is called a full septic screen. This is a series of tests designed to look for any source of serious infection. It typically includes blood tests (a full blood count to check white cell levels, a blood culture to check for bacteria in the blood, and C-reactive protein to measure inflammation), a urine sample collected via a sterile catheter or bag, and often a lumbar puncture. A lumbar puncture collects a small sample of the fluid around the brain and spinal cord to check for meningitis. The procedure sounds frightening but it is well-tolerated by newborns and provides information that cannot be obtained any other way.
Your baby will almost certainly be admitted to hospital and started on intravenous antibiotics while the results of the tests come back. This is precautionary. Starting antibiotics before the results are in means that if there is a bacterial infection, treatment is not delayed by the time it takes for cultures to grow. If all results return clear, the antibiotics are usually stopped and your baby can go home.
Group B strep and other neonatal infections
Group B Streptococcus (GBS) is the most common cause of serious bacterial infection in newborns in the UK. Around 1 in 4 pregnant women carry GBS in the vagina or gut without knowing it, and the bacteria can be passed to the baby during birth. Late-onset GBS infection (occurring between 7 days and 3 months of age) can present with fever, poor feeding, or irritability and must be treated urgently. The Group B Strep Support charity and the NHS both advise that any fever in a newborn is investigated immediately, partly because of this risk.
Most fevers in this age group turn out to be viral
It is worth saying clearly: the majority of babies under 3 months who are assessed in hospital for fever are found to have a viral infection rather than a serious bacterial one. Viruses like respiratory syncytial virus (RSV) and rhinovirus circulate throughout the year, and newborns can pick them up from family members. A viral infection in a young baby still warrants careful observation, but it does not require antibiotic treatment. The point is that you cannot tell from home whether your baby's fever is viral or bacterial, and doctors cannot either without tests. That is why every fever in this age group needs to be assessed, and why acting quickly is always the right call.
Frequently asked questions
What temperature is dangerous in a newborn?
Any temperature of 38°C (100.4°F) or above in a baby under 3 months is treated as urgent by the NHS and the AAP. You should call 111 or your GP immediately and not wait to see if it comes down on its own.
Can I wait and see if my newborn's fever goes down?
No. Do not wait and see with a baby under 3 months. Call 111 or your GP right away. In very young babies a fever can be the only sign of a serious bacterial infection that can progress rapidly. Waiting is not safe.
Why is fever in newborns treated so seriously?
Newborns have immature immune systems that cannot contain infections the way older babies and children can. They cannot localise an infection to one part of the body, so bacteria in the bloodstream can spread quickly. Serious bacterial infections are more common in this age group than in older babies, and the consequences of delayed treatment are more severe.
What tests will the hospital do for a newborn with fever?
Doctors will usually do a full septic screen. This typically includes blood tests (full blood count, blood culture, C-reactive protein), a urine sample, and sometimes a lumbar puncture to rule out meningitis. Your baby will usually be admitted and started on intravenous antibiotics while the results come back, even before an infection is confirmed.
Will my baby need antibiotics for a fever?
In most hospitals, a baby under 3 months with a fever will be started on intravenous antibiotics while test results are awaited. This is a precaution, not a certainty that there is a bacterial infection. If tests come back clear, antibiotics are usually stopped. It is better to treat and find no infection than to delay treating a real one.
How do I take a newborn's temperature accurately?
The most accurate methods for newborns are an axillary (armpit) digital thermometer or a rectal thermometer. Ear thermometers are not reliable in very young babies because the ear canal is too narrow. Strip thermometers held on the forehead are not accurate enough. A digital thermometer held in the armpit for the time specified in the instructions is the simplest reliable method at home.
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