Baby Fever: When to Worry and What to Do

Few things make a new parent’s heart race quite like touching your baby’s forehead and feeling that unmistakable warmth. Your mind starts spinning — is this serious? Should we go to the ER? Is it just teething? Take a breath. Fever in babies is incredibly common, and while it absolutely deserves your attention, it’s rarely the emergency it feels like at 2 a.m.

This guide will walk you through everything you need to know about baby fevers — from how to take an accurate temperature to recognizing the warning signs that mean it’s time to call your pediatrician or head to the emergency room.

What Counts as a Fever in Babies?

A fever is defined as a rectal temperature of 100.4°F (38°C) or higher. This is the standard used by the American Academy of Pediatrics (AAP) and pediatric emergency departments worldwide.

Here’s an important detail many parents miss: normal body temperature fluctuates throughout the day. It tends to be lower in the morning and higher in the late afternoon, so a reading of 99°F at bedtime isn’t necessarily a fever — it might just be your baby’s normal evening temperature.

Temperature by measurement method

Not all thermometers give the same reading, and not all methods are equally reliable for babies:

Rectal (most accurate for babies under 3 months): 100.4°F / 38°C = fever
Oral: 100°F / 37.8°C = fever
Axillary (armpit): 99°F / 37.2°C = fever (less reliable, good for screening)
Temporal artery (forehead): Convenient but can read slightly higher than rectal; best used as a screening tool
Tympanic (ear): Not recommended for babies under 6 months due to small ear canals

For babies under 3 months old, the AAP recommends rectal temperature as the gold standard. If you get a concerning reading from a forehead or armpit thermometer, always confirm with a rectal check before heading to the ER.

Why Do Babies Get Fevers?

Fever isn’t a disease — it’s your baby’s immune system doing exactly what it’s designed to do. When the body detects an invader like a virus or bacteria, it raises its internal thermostat to create an environment that’s less hospitable to germs.

Common causes of fever in babies include:

Viral infections — by far the most common culprit. Colds, flu, roseola, and other viruses typically cause fevers that resolve on their own within a few days.
Ear infections — often follow a cold and may cause fever along with fussiness and ear-pulling.
Urinary tract infections — sometimes the only symptom in young babies is an unexplained fever.
Vaccinations — a low-grade fever in the 24 to 48 hours after shots is a normal immune response and generally nothing to worry about.
Teething — this one is controversial. Research suggests teething may cause a very slight temperature rise (below 100.4°F) but does not cause true fevers. If your teething baby has a genuine fever, look for another cause.
Bacterial infections — less common but more serious, including pneumonia, meningitis, or bloodstream infections.
Overdressing — bundling a baby in too many layers can temporarily raise body temperature, especially in newborns.

The Age Factor: Why It Matters So Much

Your baby’s age is the single most important factor in determining how urgently a fever needs attention. Here’s why: very young infants have immature immune systems and limited ability to fight serious infections. A fever that might be completely benign in a toddler could signal something dangerous in a newborn.

Under 1 month (0–28 days)

Any fever of 100.4°F or higher is a medical emergency. Do not wait. Do not pass go. Head directly to the emergency department. At this age, babies lack the immune reserves to fight serious bacterial infections, and a fever may be the only early warning sign of conditions like meningitis or sepsis. Hospital evaluation typically includes blood work, urine tests, and sometimes a lumbar puncture.

1 to 3 months

A fever at this age still warrants immediate medical evaluation — call your pediatrician right away or go to the ER if your doctor’s office is closed. Many infants in this age group will need lab work to rule out serious bacterial infections, though the AAP’s 2021 clinical practice guideline now provides frameworks for identifying lower-risk infants who may be managed more conservatively.

3 to 6 months

Call your pediatrician for any fever of 100.4°F or higher. Your doctor will likely ask about other symptoms — how the baby is feeding, whether they’re producing wet diapers, and their overall behavior. Many fevers in this age group are viral and can be monitored at home, but your pediatrician’s input is important.

6 to 24 months

At this age, the immune system is more developed. Low-grade fevers with a clear cause (like a runny nose) can often be monitored at home. Call your doctor if the fever reaches 102°F (38.9°C) or higher, lasts more than 24 hours without improvement, or is accompanied by concerning symptoms.

Over 24 months

Focus shifts more to how the child is acting than the number on the thermometer. A toddler with a 103°F fever who’s still playing and drinking fluids is generally less concerning than one with a 101°F fever who’s limp and refusing to eat.

How to Take Your Baby’s Temperature

Taking a rectal temperature sounds intimidating, but it’s simpler than you’d think:

Use a digital thermometer with a flexible tip designated for rectal use.
Apply a small amount of petroleum jelly to the tip for comfort.
Lay your baby face-down on your lap or face-up on a firm surface.
Gently insert the tip about half an inch to one inch into the rectum.
Hold still until the thermometer beeps (usually 10 to 30 seconds).
Clean the thermometer thoroughly with rubbing alcohol or soap and water after each use.

Pro tip: Label your rectal thermometer clearly so it doesn’t end up in someone’s mouth. Many parents use a piece of colored tape to distinguish it.

How to Help Your Baby Feel Better

When your baby has a fever and your pediatrician has confirmed it’s safe to manage at home, here’s how to provide comfort:

Fever-reducing medication

Acetaminophen (Tylenol) can be given to babies 2 months and older. Always dose by weight, not age, and use the dosing syringe that comes with the product.
Ibuprofen (Motrin, Advil) is approved for babies 6 months and older. It can be alternated with acetaminophen if one medication alone isn’t providing enough relief — but check with your pediatrician about the timing first.
Never give aspirin to children or teenagers. It’s linked to a rare but serious condition called Reye’s syndrome.

Comfort measures

Keep your baby lightly dressed. Resist the urge to bundle them up — overdressing traps heat.
Offer extra fluids. For babies under 6 months, that means breast milk or formula. For older babies, small sips of water or an electrolyte solution can help.
Give a lukewarm bath. Not cold — cold water can cause shivering, which actually raises body temperature. Lukewarm water helps heat dissipate gently.
Keep the room comfortable. Around 68–72°F (20–22°C) is ideal.
Let them rest. Fever is energy-intensive. Your baby may sleep more than usual, and that’s okay.

What NOT to do

Don’t use rubbing alcohol baths — the fumes are dangerous and alcohol can be absorbed through the skin.
Don’t use ice baths or very cold water.
Don’t starve a fever — continue regular feedings.
Don’t alternate fever medications without your doctor’s guidance on timing and dosing.

Febrile Seizures: Scary but Usually Harmless

About 2 to 5 percent of children between 6 months and 5 years will experience a febrile seizure — a convulsion triggered by fever. Watching your child have a seizure is terrifying, but here’s what you should know:

Most febrile seizures last less than 5 minutes and stop on their own.
They do not cause brain damage or increase the risk of epilepsy.
They tend to run in families — if you or your partner had one as a child, your baby is more likely to as well.

If your baby has a febrile seizure, lay them on their side on a safe surface, don’t put anything in their mouth, note the time, and call 911 if it lasts longer than 5 minutes. Even if the seizure stops quickly, call your pediatrician afterward for guidance.

When to Seek Emergency Care

Head to the emergency room or call 911 if your baby with a fever:

Is under 1 month old with any fever of 100.4°F or higher
Is between 1 and 3 months old and you cannot reach your pediatrician
Has a fever above 104°F (40°C) at any age
Is unusually limp, difficult to wake, or unresponsive
Has a rash that looks like purple or red spots that don’t fade when you press on them
Has difficulty breathing — you may notice the skin pulling in between the ribs or the nostrils flaring
Is having a seizure lasting more than 5 minutes
Seems to be getting sicker despite fever medication
Has a stiff neck or bulging soft spot
Refuses to drink and shows signs of dehydration (no wet diapers for 8+ hours, no tears when crying, sunken eyes)

Frequently Asked Questions

Can I use a forehead thermometer for my newborn?

You can use it as a screening tool, but for babies under 3 months, always confirm a concerning reading with a rectal temperature. Forehead thermometers can be affected by sweating, ambient temperature, and technique.

My baby had vaccines today and now has a fever. Is this normal?

Yes — a low-grade fever after vaccinations is a normal immune response and typically resolves within 24 to 48 hours. You can give acetaminophen if your baby is uncomfortable (for babies 2 months and older). Contact your pediatrician if the fever exceeds 102°F or lasts more than 48 hours.

Does a higher fever mean a more serious illness?

Not necessarily. The height of the fever doesn’t reliably predict the severity of the illness. A baby with a 101°F fever from a urinary tract infection might need antibiotics, while a baby with a 104°F fever from roseola will recover on their own. How your baby looks and acts matters more than the exact number.

Should I wake my sleeping baby to give fever medication?

Generally, no. Sleep is healing. If your baby is sleeping comfortably, let them rest. You can check their temperature when they wake naturally. The exception is if your pediatrician has specifically instructed you to give medication on a schedule.

How long is too long for a baby to have a fever?

For babies under 3 months, any fever warrants prompt evaluation regardless of duration. For older babies, contact your pediatrician if a fever persists for more than 3 days, if it keeps returning after initially resolving, or if your baby seems to be getting worse rather than better.

Sources

American Academy of Pediatrics. “Evaluation and Management of Well-Appearing Febrile Infants 8 to 60 Days Old.” Pediatrics, vol. 148, no. 2, 2021.
Sullivan, J.E., and Farrar, H.C. “Fever and Antipyretic Use in Children.” Pediatrics, vol. 127, no. 3, American Academy of Pediatrics, 2011, pp. 580–587.
HealthyChildren.org (AAP). “Fever and Your Baby.” American Academy of Pediatrics, updated 2024.
World Health Organization. “Pocket Book of Hospital Care for Children: Guidelines for the Management of Common Childhood Illnesses.” 2nd ed., WHO, 2013.
Subcommittee on Febrile Seizures. “Febrile Seizures: Guideline for the Neurodiagnostic Evaluation of the Child.” Pediatrics, vol. 127, no. 2, 2011, pp. 389–394.