A fever may mean that a baby’s body is fighting off an infection. This is a sign of a healthy immune system. However, because newborns have more vulnerable bodies, a baby fever can signal a serious infection.

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Understanding the temperature of fever in babies and what can cause fever can help with knowing how to care for the baby and when to seek medical advice.

This article examines the causes of fever in babies, what it means, and when a parent or caregiver may want to take the baby to a doctor. It also discusses how to care for a baby with a fever.

When taking a baby’s temperature, a rectal thermometer generally gives the most accurate results.

What is fever in babies?

A typical temperature in a child under the age of 12 months is an average of 98.6°F (37°C) when using a rectal thermometer. In the morning, this temperature can be as low as 96.8°F (36°C) and go up to 100.3°F (37.9°C) later in the day. This is a normal range for an infant at this age.

Fever in a child depends on the method of taking the temperature:

  • above 100.4°F (38°C) using a rectal thermometer
  • above 100°F (37.8°C) using an oral thermometer, however, this method is not accurate in infants
  • above 99°F (37.2°C) using an armpit thermometer

What does fever mean?

By itself, a fever does not necessarily signal a serious illness. If the baby is behaving as they would, they are likely to be fine. However, if a baby under 3 months of age has a rectal temperature of 100.4°F (38°C) or higher, a parent or caregiver should call a doctor.

Additionally, the severity of a fever does not always indicate how unwell a child is at the time. The body temperatures of babies can rise for many reasons other than illness, including extended crying, sitting in the hot sun, or spending time playing. Their temperature may also slightly rise when they are teething. None of these things causes a fever.

A baby’s body is also less able to regulate temperature than an adult body, meaning it can be more difficult for them to cool down during a fever. Their bodies are naturally warmer than an adult’s body because they are more metabolically active, which generates heat.

A fever is a symptom of an illness, not the illness itself. Fever occurs due to the immune system fighting against infections, which can be bacterial or viral. Viral infections are much more common.

Common causes of fevers in babies include:

  • viral infections, including the common cold, the flu, roseola, or respiratory infections, such as respiratory syncytial virus or viral croup
  • ear infections
  • pneumonia, which can be viral or bacterial
  • meningitis, which can be viral or bacterial and is a very serious infection of the brain and spinal cord
  • A fever of 100.4°F (38°C) or higher before the age of 3 months. At this age, any bacterial infection can progress quickly and lead to sepsis, so an infant needs immediate medical care.
  • A fever after a vaccine that can occur within 12 hours after the shot and lasts for 2–3 days.
  • Rarely, heat-related illnesses can cause high temperatures in babies. Babies are less effective at controlling their body temperature than adults, making them more vulnerable to very hot weather. Dressing babies in weather-appropriate clothing, keeping them out of the hot sun, and keeping them indoors when the weather is very hot will help regulate their body temperature.
  • urinary tract infection, a bacterial infection that can sometimes cause fever in babies

Despite common beliefs, teething does not cause a fever.

Fevers are a sign that the baby’s immune system is fighting infection, so the fever itself is not dangerous. What may put the baby at risk is the underlying infection, if it is serious.

Should parents be worried?

Some parents and caregivers may worry that fevers are dangerous. However, except in rare cases, these fevers themselves are rarely serious.

Fevers of up to 105°F (40.5°C) are common in babies and children whose temperatures often get much higher than an adult’s temperature. Where there is a viral infection causing this fever, they may just need to run their course. Some bacterial infections may require treatment with antibiotics, but the fever itself is just a symptom.

Additionally, treating the fever will not make the infection go away. Instead, parents and caregivers should simply carefully monitor the child for signs of complications.

However, a doctor needs to examine babies younger than 3 months for any signs of fever. This is to check for underlying conditions, as certain infections are more common and can be more dangerous in newborns.

Fever complications

Some parents or caregivers may hear about fevers causing brain damage. However, this can only happen if the temperature rises above 107°F (41.6°C), which is very rare. When an infant’s temperature is below this, there is no need to take drastic measures, such as ice baths, to lower the child’s fever.

For 2–5% of children aged between 6 months and 5 years, a fever can cause a seizure, which can be worrying but is not typically harmful. Doctors call them febrile seizures.

Febrile seizures do not cause brain damage or increase a child’s risk of epilepsy. Even long seizures, or those that last longer than 15 minutes, usually have a good outcome. However, prolonged seizures may mean a child is more likely to develop epilepsy.

The biggest risk of febrile seizures is that a child may fall, hit their head, or suffer a similar injury. Therefore, parents and caregivers should monitor children during a seizure to prevent injury and call 911 or emergency services for any seizures in a baby that continue for longer than 5 minutes.

The American Academy of Pediatrics (AAP) recommends that parents and caregivers make their babies comfortable if they have a fever, rather than focusing on lowering their temperature.

To look after a baby with a fever, parents and caregivers can:

  • Monitor the baby’s activity level and overall comfort: Babies who seem happy, alert, and comfortable may not need treatment.
  • Ensure the baby remains well hydrated: Fever increases the risk of dehydration, so parents, caregivers, and nurses should offer milk or formula on demand. Older babies should also drink plenty of water. In some cases, a healthcare professional may recommend using an electrolyte drink to help with dehydration.
  • Monitor the baby for signs of dehydration: This can include not urinating as often as usual, sunken eyes, chapped lips, or very dry or pasty-looking skin.
  • Avoid waking a sleeping baby to administer anti-fever medication: Unless a doctor instructs to do this.
  • Administer anti-fever medication under a doctor’s supervision: People can give this medication if the infant is in pain or uncomfortable from the fever. The baby’s weight determines the dose, so follow the label instructions carefully. Call a doctor before giving new medication to a baby, especially a sick one.
  • Help limit the spread of infections: Do not send a sick baby to daycare or take them to places where babies or other vulnerable people may be, as this can spread infections.

Call a doctor or seek medical care if a baby has a fever and one of the following:

  • The baby is inconsolable, lethargic, or seems very sick.
  • A fever lasts longer than 24 hours in a baby younger than 2 years old without other symptoms.
  • The fever rises above 104°F (40°C).
  • The child still seems sick with medication.
  • The baby is taking antibiotics but does not seem better within a day or two.
  • The baby has signs of dehydration, including dry lips or a sunken soft spot on top of their head.
  • The baby has a weak immune system for a separate reason.
  • The baby is younger than 3 months old.

Go to the emergency room for a fever if a baby:

  • is a newborn
  • has a seizure for the first time
  • has a seizure that lasts longer than 15 minutes
  • has a fever rising to 107°F (41.6°C) or higher

A fever in a newborn may be a sign of a serious medical condition. Newborns are more vulnerable to infections, so it is important to take any signs of infection seriously. Call a doctor if a newborn has a fever or other signs of illness.

One concern with newborns is respiratory illness. Newborns breathe more through their noses than older infants and children, so congestion can make breathing appear more difficult. They also have smaller airways.

A lack of oxygen can seriously injure a newborn. If a baby has trouble breathing, call a doctor, even if their fever goes down.

Signs that a newborn is having trouble breathing include:

  • wheezing or grunting
  • flaring the nostrils when breathing
  • white or blue skin, especially around the nail beds or on the mouth or tongue, though this may be harder to detect in infants with darker skin.
  • pulling in the muscles around the ribs when breathing

If a baby has breathing problems and a fever, a parent or caregiver should take them to the emergency room immediately.

Young children and babies sometimes experience fevers, but this is usually not a cause for concern when infants are behaving as expected.

Parents and carers can use a child’s behavior as a cue. If a baby seems fine but has a fever, the illness is probably a minor one that will soon pass.

Lethargy, excessive crying, and other signs of serious illness are important to address, even if a child’s fever is fairly low. A fever means that the immune system is working hard to fight an infection.

Parents and caregivers do not need to treat the fever itself, but they can comfort the baby and treat the symptoms instead. If they are unsure whether a baby’s symptoms are serious, they should consult with a healthcare professional.