When Your Newborn is Sick:
Fever, Taking Your Baby’s Temperature and When to Seek Medical Help

As a new parent, it can be worrying when your baby seems unwell. Fortunately, many illnesses in newborn babies are mild and improve with time. However, because newborn babies can become seriously ill very quickly, it is important to recognise the warning signs and know when to seek medical attention.

Trust your instincts. If you think your baby is seriously unwell, seek medical advice immediately.

What is a Fever?

A fever is the body’s natural response to fighting an infection.

In newborn babies and young infants, a fever is taken very seriously because their immune systems are still developing.

A temperature of 38°C or higher in a baby under 3 months of age requires urgent medical assessment, even if your baby otherwise appears well.

A fever is not an illness itself—it is a sign that your baby’s body may be fighting an infection.

Signs Your Newborn May Be Unwell

Babies cannot tell us when they feel sick, so it is important to look for changes in their behaviour.

Contact your GP, maternity hospital or out-of-hours GP service if your baby:

Has a temperature of 38°C or higher.
Is unusually sleepy or difficult to wake.
Refuses feeds or feeds much less than usual.
Has fewer wet nappies.
Cries constantly or has a weak or unusual cry.
Seems floppy or unusually limp.
Is very irritable and difficult to comfort.
Is breathing faster than normal.
Is vomiting repeatedly.
Looks pale, mottled or grey.
Is generally not behaving like themselves.

Even if your baby does not have a fever, you should seek medical advice if they appear unwell.

How to Take Your Baby’s Temperature

If you think your baby has a fever, use a digital thermometer.

The HSE recommends taking your baby’s temperature under the armpit (axillary temperature).

Taking an Armpit Temperature
Make sure your baby’s armpit is dry.
Place the tip of the digital thermometer high in the centre of the armpit.
Gently hold your baby’s arm against their body.
Wait until the thermometer beeps.
Read and record the temperature.

Always follow the manufacturer’s instructions for your thermometer.

Which Thermometers Should I Use?

The HSE recommends:
Digital thermometer (under the armpit)

The HSE does not recommend using:

Glass mercury thermometers.
Forehead strip thermometers.
Dummy (pacifier) thermometers.

Ear thermometers are generally not recommended for young babies because their ear canals are very small and readings may not be accurate.

What Temperature is Considered a Fever?

For babies under 3 months:

Normal: usually below 38°C.
Fever: 38°C or above.

Do not wait to see if the temperature comes down on its own.

A baby younger than 3 months with a temperature of 38°C or higher should be assessed urgently by a doctor.

What Should I Do if My Baby Has a Fever?

If your newborn develops a fever:

Remove excess clothing or blankets.
Keep the room comfortably cool.
Continue feeding regularly.
Offer breastfeeds or formula feeds as normal.
Monitor the number of wet nappies.
Contact your GP, maternity hospital or out-of-hours GP service immediately if your baby is under 3 months old.

Looking for Signs of Dehydration

When babies are unwell they can become dehydrated quickly.

Signs of dehydration include:

Fewer wet nappies.
Dry mouth or lips.
No tears when crying (after the newborn period).
Sunken eyes.
Sunken soft spot (fontanelle).
Being unusually sleepy.

Seek medical advice promptly if you think your baby may be dehydrated.

Breathing Problems

Newborn babies normally breathe a little irregularly, but you should seek urgent medical advice if your baby:

Is breathing very fast.
Is struggling to breathe.
Is grunting.
Has their nostrils flaring.
Is sucking in under the ribs or around the neck.
Stops breathing for more than a few seconds.
Has blue lips or tongue.

Breathing difficulties should always be assessed urgently.

Vomiting and Diarrhoea

Small amounts of milk coming back up after feeds (posseting) is common.

Seek medical advice if your baby:

Vomits repeatedly.
Vomits forcefully after every feed.
Vomits green fluid.
Vomits blood.
Has persistent diarrhoea.
Shows signs of dehydration.
Rashes

Many newborn rashes are harmless.

However, seek urgent medical attention if your baby has:

A rash that does not fade when you press a glass against it.
A rash together with a fever.
Blisters or rapidly spreading spots.
A rash and appears generally unwell.
Trust Your Instincts

Parents often notice that “something isn’t right” before obvious symptoms develop.

If your newborn:

Is unusually sleepy.
Is feeding poorly.
Is difficult to wake.
Is not responding normally.
Simply doesn’t seem like themselves,

contact your GP, maternity hospital or emergency services without delay.

It is always better to have your baby checked than to wait.

When to Call 112 or 999 Immediately

Call emergency services immediately if your baby:

Has difficulty breathing.
Stops breathing.
Has blue lips or tongue.
Is very floppy or unresponsive.
Has a seizure.
Cannot be woken.
Has a rash that does not fade when pressed.
Has pale, grey or mottled skin.
Has a high-pitched, weak or unusual cry together with signs of serious illness.

Key Points to Remember
A temperature of 38°C or higher in a baby under 3 months is a medical emergency and requires urgent assessment.
Use a digital thermometer under your baby’s armpit to check their temperature.
Continue offering regular feeds while your baby is unwell.
Watch for signs of dehydration, breathing difficulties and unusual drowsiness.
Trust your instincts—if you are worried about your baby, seek medical advice.

Remember, most newborn illnesses are mild, but young babies can become seriously ill very quickly. Early assessment and treatment can make a significant difference.