Fever in a Baby Under 3 Months: What to Do
A fever of 38.0°C or above in a baby under 3 months needs urgent medical assessment, even if the baby looks well. Do not give fever medicine first. Go to your nearest 24-hour Children's Emergency. A young baby's immune system cannot wall off a serious infection the way an older child's can.
What to do right now
If your baby is under 3 months old and has a temperature of 38.0°C or above, treat it as urgent. The single most useful thing you can do is get them assessed quickly.
Call 995 now if your child has a fever and any of these:
- !Any seizure, however short
- !A seizure lasting more than 5 minutes (give rectal diazepam if a doctor has prescribed it, and still call)
- !Struggling to breathe (skin pulling in under the ribs, nostrils flaring, or grunting), pauses in breathing, or lips turning blue or grey
- !Cannot be woken, or floppy and not responding to you
- !A rash of small red or purple spots that does not fade when you press a glass on it
- !Swelling of the lips, tongue, or face, with any difficulty breathing
- !Cold, clammy, or mottled skin, with drowsiness or restlessness
Go to the nearest 24-hour A&E or children's emergency now if:
- !A bulging soft spot on top of the head
- !A weak, high-pitched or constant cry
- !Feeding much less than usual, or fewer wet nappies
- !Breathing faster than usual
- !A temperature below 36.0°C
- !A baby under 3 months with a temperature of 38.0°C or above
- !A first febrile seizure, even if it has stopped
- !After a seizure: confused, still drowsy after 30 minutes, or more than one seizure in a day
- !A stiff neck, or crying in pain when bending the head forward
- !Unusually drowsy or hard to wake, but can be woken
- !Dengue warning signs as the fever comes down: tummy pain, repeated vomiting, bleeding gums or nose, or very tired
- !No urine or wet nappy for 12 hours or more
- !No tears when crying, sunken eyes, or a very dry mouth
- !Cannot keep any fluids down
- !Green vomit
- !A temperature above 40°C that is not coming down, and your child seems unwell
Do not give fever medicine before your baby is assessed. Bring any feeding notes and the time the fever was first noticed.
Why fever is different at this age
In an older child, a fever is usually the body handling a common virus, and how the child behaves tells you most of what you need to know. In a baby under 3 months, that reasoning does not hold. Their immune system is still immature and cannot wall off an infection to one area. An infection that would stay minor in a toddler can spread quickly in a newborn.
This is why a young baby with a fever can look settled and still be seriously unwell. Doctors do not rely on appearance alone at this age. They examine, and often run tests, to find or rule out a bacterial infection early, because early treatment changes the outcome.
The numbers that matter
A fever at this age. Go to Children's Emergency for assessment, even if your baby looks well.
Borderline. Remove extra layers, wait 20 minutes, and recheck. If it stays up or your baby seems off, get medical advice the same day.
Too low. A young baby who cannot stay warm needs urgent assessment, the same as a fever.
These thresholds apply to babies under 3 months specifically. Older babies and children have more room to monitor at home.
What not to do
- Do not give paracetamol or ibuprofen before your baby is assessed. It can mask the picture a doctor needs to see, and ibuprofen is not used in babies this young.
- Do not wait overnight to see if it settles. Fever at this age is assessed promptly, at any hour.
- Do not sponge with cold water or use a fan to force the temperature down. It causes distress without helping.
- Do not assume a normal-looking baby is fine. Appearance is not a reliable guide under 3 months.
Taking an accurate temperature
For a baby under 3 months, the practical home method is an axillary (armpit) reading with a digital thermometer:
- Make sure the armpit is dry and your baby has not been bundled in blankets just before.
- Place the thermometer tip in the centre of the armpit, against skin.
- Hold the arm gently against the body until it beeps.
- If the reading is borderline, remove a layer, wait 20 minutes, and check again.
Ear thermometers are less reliable in the small ear canals of young babies. Forehead thermometers are convenient for a quick check but can read low. If a quick method shows a fever, confirm with an armpit reading, but do not delay getting help to chase a perfect number.
→ How to take your child's temperature: ear, forehead, and armpit compared
What to expect at the hospital
Knowing what usually happens can take some of the fear out of going in. For a young baby with a fever, the team will typically:
- Examine your baby fully and check feeding, alertness, and breathing.
- Measure temperature, heart rate, and breathing rate.
- Often take blood and urine samples to look for infection.
- Often admit very young babies, especially those under 1 month, for tests and antibiotics while results come back.
Tests in this age group are a normal, careful part of assessment, not a sign that something terrible is assumed. Admission for tests and antibiotics is routine caution in very young babies, not a sign something is badly wrong. Older babies may be checked and sent home. The point of going in is to catch the small number who need treatment before they become very unwell.
→ Taking an accurate temperature at home
→ Fever at night: when should parents worry?
Frequently asked questions
What temperature counts as a fever in a baby under 3 months?+
A temperature of 38.0°C or above measured under the arm is a fever in a baby this young, and it needs urgent medical assessment. A reading between 37.5°C and 37.9°C is borderline: recheck after removing extra layers, and if it stays up or the baby seems unwell, get medical advice the same day.
My baby is under 3 months with a fever but looks fine. Do I still need to go in?+
Yes. A young baby can have a serious infection while still feeding and looking settled, because their immune system cannot localise infection the way an older child's can. Looking well does not rule out a problem at this age. Go to your nearest Children's Emergency for assessment.
Should I give my baby paracetamol before going to the hospital?+
No. Do not give fever medicine to a baby under 3 months before they are assessed. Lowering the temperature can mask signs the doctor needs to see, and paracetamol dosing for very young infants should be decided by a clinician. Bring the baby in as they are.
How do I take an accurate temperature on a newborn?+
For babies under 3 months, an axillary (armpit) reading with a digital thermometer is the practical method most parents use at home. Place the tip in the centre of a dry armpit, hold the arm gently against the body, and wait for the beep. Ear thermometers are less reliable in very small ear canals.
What if my baby is too cold instead of hot?+
A temperature below 36.0°C in a young baby can also signal illness and needs medical assessment, the same as a fever. Babies this age do not always mount a fever when they are unwell. A low temperature, poor feeding, or unusual floppiness all warrant urgent review.
Why is fever treated so much more seriously at this age?+
In the first 8 to 12 weeks of life, the immune system is still immature and a baby is more vulnerable to bacterial infections that can progress quickly. Doctors investigate fever in this group carefully, often with tests, because catching a serious infection early makes a large difference.