Baby Fever: What It Is, Common Causes & How to Treat

Updated in February 2022

Increased body temperature in a baby is considered to be a fever if the baby has an axillary temperature over 37.5ºC (or 99.5ºF) or a rectal temperature of 38.2ºC (or 100.7ºF). Anything below this temperature is generally not of concern. 

A fever is not always a sign of illness, as it can be caused by prolonged exposure to heat or excess clothing, however it can be of concern if the baby is also fighting an infection, like a cold or flu.

If your baby has a fever, you should note if other occurrences that could have triggered it, like teething or a recent vaccination, took place. You should also monitor whether the baby is eating and sleeping well. A cool, wet towel on the forehead can help to lower a fever.  

A fever over 41.5ºC or (106.7ºF) that is left untreated may lead to brain damage.

Imagem ilustrativa número 3

Common causes

Elevated body temperature can indicate that the body is trying to fight off a foreign invader. The most common situations that can cause a fever in babies are: 

  • Teething: This generally occurs after the 4th month. Other symptoms include swollen gums, drooling and always having a hand in the mouth. 
  • Vaccine reaction: A reaction can occur a few hours after injection, A fever is the most common vaccine reaction. 
  • After a cold or flu: If a fever emerges after a cold or flu infection, sinusitis or an ear infection may be suspected. The baby not have any phlegm or cold symptoms, but the internal tissues of the nose and throat may be inflamed, causing a fever. 
  • Pneumonia: This infection causes flu-like symptoms that become more intense, resulting in fever and difficult breathing.  
  • Urinary tract infection (UTI): Low grade fever (up to 38.5ºC or 100.7ºF rectally= can be one of the only signs of a UTI in children less than 2. Some babies may also experience vomiting, diarrhea, abdominal pain and loss of appetite.  
  • Dengue: This is most common in the summer, especially in affected areas. It causes symptoms like fever, loss of appetite, irritability and sleepiness.
  • Chicken pox: It is associated with fever, itchy skin blisters, loss of appetite and abdominal pain.
  • Measles: This infection causes a fever that lasts 3 to 5 days as well as coughing, runny nose, conjunctivitis and dark patches of skin.  
  • Scarlet fever: Fever and a sore throat typically occur, as well as a swollen tongue (similar to a raspberry) and small red skin patches that can peel. 
  • Erysipelas: This skin infection causes fever, chills, and pain, redness and swelling in the affected area 
  • Increased body heat: Prolonged sun exposure or wearing too many layers of clothing can lead to increased body temperature. 

If you suspect your baby has a fever or if they feel warm to the touch, you should measure their temperature. Monitor for any other signs or symptoms that can help to identify the reason for the fever. If in doubt, consult your doctor, especially if your baby is less than 3 months of age. 

How to measure temperature

To measure a baby's temperature, you should place the metallic part of the thermometer in the baby's armpit and keep it in place for 3 minutes (or until it beeps, if it is digital). Then remove the thermometer and verify the temperature.

Rectal temperatures are another, even more accurate method. It is important to remember, however, that rectal temperatures will be slightly higher than an oral or axillary temperature. For example, if the baby has a 37.8ºC or 100ºF axillary temperature, they will likely have a 38.8ºC or 101.8ºF rectal temperature. Therefore, when monitoring temperature or a fever, you should always use the same place to check it. 

While there are other digital thermometers available that can read temperature on the forehead or inside the ear, these are not recommended for babies under the age of 2. Although convenient, these methods are not as accurate for this age group.

Treatment options

When a fever is confirmed, the goal is to keep the baby comfortable and hydrated. Medication may not always be necessary, keeping in mind that a fever is the body's natural way to fight off an infection and maintain homeostasis. 

Lowering body temperature naturally

Before offering medication, you can try lowering a baby's fever by:

  • Ensuring the environment is not too hot. Turn on a fan or air conditioning if necessary. 
  • Removing extra clothes layers or changing to a looser, fresh outfit.
  • Offering cold fluids every 30 minutes when awake. 
  • Giving the baby a bath in lukewarm water, making sure to avoid using cold water. The bath temperature should be close to 36ºC or 96.8ºF, which is closer to normal skin temperature. 
  • Placing lukewarm, wet towels on the baby's head, armpits and neck.

If the fever does not lower within 30 minutes, you should advise your doctor, especially if the baby is very irritable, crying a lot or lethargic. 

Using medication

Medications should only be used with direction from the doctor. The recommended medications are acetaminophen and ibuprofen. 

If fever occurs with inflammation, the doctor may advise you to alternate use of both acetaminophen and ibuprofen ever 4, 6 or 8 hours. Dosing will vary depending on the child's weight, therefore you should be very careful when administering. 

The doctor may also prescribe antibiotics if the fever is caused by a viral or bacterial infection. 

Normally, use of medication is only recommended for fever over 37.5ºC (or 99.5ºF). Fevers are a natural defense mechanism to fight off viruses and bacteria, and therefore medication should not be given immediately for a low-grade fever.

Fevers will usually resolve within 3 days of a viral infection and 2 days of a bacterial infection after using antibiotics.. 

When to seek medical attention

You should go to the emergency room or consult your doctor immediately when: 

  • The baby is less than 3 months of age
  • The fever is over 38ºC (or 100.4ºF) and it increases rapidly to 39.5ºC (or 103.1ºF), as this may indicate a bacterial infection
  • If the baby has no appetite, refuses to breastfeed or bottle feed, is sleeping more than usual, and when awake, shows intense irritability. These are signs of a more serious infection. 
  • Skin rashes or patches
  • Other symptoms like increased crying or moaning. 
  • Lethargy or a new difficulty with sitting or walking
  • Difficulty breathing
  • If the baby refuses more than 3 consecutive meals 
  • Signs of dehydration 
  • If the baby does not sleep more than 2 consecutive hours in the day or night, as a fever usually results in increased drowsiness. 

If the baby has a seizure and starts to tremor, remain calm and lay the baby on the floor to their side, making sure to protect their head. Remove any pacifiers, or food if possible, from the mouth. A febrile seizure normally lasts about 20 seconds and will happen just once. It is not of major concern. If the seizure lasts for over 2 minutes, you should take the baby to the hospital. 

When speaking to the doctor, make sure you confirm the baby's age and when the fever started. You should also state whether the fever has been continuous, or if the temperature has been fluctuating, as these details can help the doctor reach a diagnosis.