Fevers 101

When a child is ill, parents feel terrible too: sick with worry, uncertainty, fear. Fevers can be especially scary. How high is too high? When should I call the doctor? Should I give my baby medication? The following guidelines are designed to give you some peace of mind as you nurse (in some cases literally) your child back to health.

What is a fever? 

A fever itself is not a disease or an infection—it signals your body’s response to one. When you get a viral or bacterial infection, your body responds by raising its temperature, which in turn switches on its immune system. Fevers increase the release and activity of white blood cells, which fight germs, making a good case for not treating low fevers but letting them do their job instead. 

Our body temperatures naturally fluctuate throughout the day: higher in the late afternoon and early evening, lower between midnight and early morning. Children’s temperatures vary by age (infants often have higher base temperatures). Your child has a fever if his oral temperature is over 99.5°F (37.5°C); rectal temperature is over 100.4°F (38°C); axillary (under the armpit) temperature is over 99°F (37.2°C); or tympanic (in the ear) temperature is over 100.4°F (38°C). According to the American Academy of Pediatrics (AAP), fevers above 105°F (40.6°C) are considered dangerous and require prompt medical attention. In addition to monitoring your child’s temperature, you should pay close attention to other symptoms.

How to take your child’s temperature 

Mercury thermometers were once the mainstay of thermometer technology, but digital thermometers now dominate the marketplace. A recent study by physicians at Children’s Hospital Boston reinforced the fact that glass thermometers are a safety hazard due to broken glass and mercury. 

Digital thermometers are specifically designed to check your child’s rectal, oral, axillary, or tympanic temperature and are placed in your child’s rectum, mouth, armpit, or ear. Some digital thermometers are versatile and can be used for rectal, oral, or axillary purposes, while others are made for use in the rectum or ear only.

For newborns–3 months: 

  • Use a rectal thermometer. 
  • Rectal thermometers can be used up to age 5. 
  • There are some concerns about inserting a rectal thermometer too far and perforating (causing a hole in) the rectum. Some thermometers have safeguards, like a wide handle, designed to keep this from happening. When taking a rectal temperature, insert the thermometer only ½–1 inch into the anal opening and hold it loosely in place.

For babies 3 months–3 years: 

  • Use a rectal, axillary, or tympanic thermometer. 
  • A tympanic thermometer requires precision and proper use. You might want to get some instructions from your baby’s doctor or nurse before using a tympanic thermometer. 
  • An axillary temperature is the least accurate of the three temperatures. An axillary temperature can be as much as two degrees lower than a rectal temperature.

For children 4–5 years: 

  • Use a rectal, oral, axillary, or tympanic thermometer. 
  • Children can begin holding an oral thermometer under their tongues for the required amount of time around this age. 
  • To ensure accuracy, wait about 30 minutes after meals, snacks, or drinks to take an oral temperature. 
  • Wait at least 20 minutes after bathtime to take your child’s temperature, since bathing can affect any temperature reading.

When to call your doctor 

The presence of other symptoms play a part in helping parents decide whether to call their child’s health care provider when their child has a fever. Parents may choose to watch and wait if their older child with a higher fever is still able to play and interact normally. But a lower fever accompanied by other symptoms may signal a need for immediate attention.

Call you child’s health care provider right away if any of the following occur: 

  • 2-month-old or younger with a rectal temperature of 100.4°F (38°C) or higher 
  • 3­- to 6-month-old with a temperature of 101°F (38.3 C) or higher 
  • 6-month-old or older with a temperature of 103°F (39° C) or higher 
  • Sore throat, earache, cough, skin rash, painful urination, stiff neck, diarrhea, vomiting, or seizure 
  • Behavioral change—fussier than usual, sleepier than usual, confused, unresponsive, lethargic 
  • High fever for more than 24 hours, even if there is no obvious cause 
  • History of febrile (fever induced) seizures

Difficulty breathing, swallowing, or awakening—call 911 immediately. 

Finally, parents should feel comfortable calling their child’s health care provider anytime they have a question or concern.

How to treat a fever at home 

Fevers typically last 1–2 days. During this time, encourage (but don’t force) your child to drink liquids, as a means of replacing the fluid lost through fever-induced sweating. Dress your child in less clothing—bundling your child, even if she complains of being cold, can cause a higher fever. If your child has chills, use a thin blanket rather than more clothes to keep her comfortable. Try other home remedies to relieve fever such as a sponge bath using lukewarm water (not cold—if your child begins shivering, take her out of the bath). Older children who are active can simply play in the tub. 

You can give acetaminophen (Tylenol) to children older than 3 months of age and ibuprofen (Advil, Motrin) to children older than 6 months of age. Before administering any medications, discuss the proper dosage with your child’s health care provider, as it’s based on your child’s weight, not age. Never give your child aspirin. Anyone under the age of 21 should not take aspirin due to its association with Reye’s syndrome (a potentially fatal disease). 

Finally, if you breastfeed, keep breastfeeding! Research shows that breastfed babies are less likely to have fevers after vaccinations. Regardless of the source of fever, breastfeeding delivers anti-inflammatory properties, antibodies, and comfort to your ill child when she needs it the most.

Last updated September 25, 2021

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