Baby's First Fever: A Parent's Guide on When to Worry and When to Wait
A fever in a baby can be terrifying for parents, but it's one of the most common reasons for a visit to the pediatrician. A fever itself isn't an illness; it's a sign that the body is fighting one. This guide will help you understand what to do when your baby's temperature rises.
What is Considered a Fever in a Baby?
A fever is a rectal temperature of 100.4°F (38°C) or higher. Always use a digital rectal thermometer for the most accurate reading in infants. A baby's first fever can be scary, but understanding the basics is key.
- Low-grade fever: 100.4°F - 102°F (38°C - 38.9°C)
- High fever: Above 102°F (38.9°C)
How to Take Your Baby's Temperature Accurately
The method matters as much as the number, because the 100.4°F (38°C) fever threshold is based on a rectal reading. Different methods can read a little higher or lower, so always tell your doctor which one you used.
- Rectal (the gold standard under 3 years): the most accurate reading for a baby. Use a dedicated digital rectal thermometer, a small dab of petroleum jelly, and insert only about half an inch — never force it.
- Forehead (temporal artery): a quick, non-invasive option that works well at any age when used exactly as the instructions describe.
- Armpit (axillary): easy but the least accurate — it tends to read low, so treat it as a rough screen and confirm a borderline result rectally.
- Ear (tympanic): not recommended under 6 months, because a small ear canal makes readings unreliable.
Skip old glass mercury thermometers and don't rely on forehead strips or a hand on the skin — neither is accurate enough to act on. Whatever method you use, note the reading, the time, and the method so you can follow the trend across the day instead of reacting to a single number.
When to Call the Doctor Immediately
A baby's age is the most critical factor in determining the urgency of a fever.
For Babies Under 3 Months
Call your doctor right away for any rectal temperature of 100.4°F (38°C) or higher. According to health experts like the Mayo Clinic, a fever in a newborn can be a sign of a serious infection. Do not give any fever-reducing medication unless instructed by your doctor.
For Babies 3 to 6 Months
Call your doctor if the fever reaches 102°F (38.9°C) or higher, or if a lower fever is accompanied by other symptoms like:
- Unusual irritability or sleepiness
- Refusing to feed
- Signs of dehydration (fewer wet diapers, no tears when crying — our free diaper calculator shows how many wet diapers are typical by age)
- A new rash (see our visual guide to common baby rashes)
For Babies Over 6 Months
A high fever isn't always a sign of a serious problem. The more important indicator is your baby's behavior. Call your doctor if:
- The fever is over 104°F (40°C) repeatedly.
- The fever lasts for more than 3-5 days.
- Your baby seems very sick, lethargic, or inconsolable.
- The fever doesn't come down with medication.
How to Comfort a Feverish Baby
Your goal is to make your baby comfortable, not necessarily to eliminate the fever completely.
- Offer Fluids: Give plenty of breast milk or formula to prevent dehydration.
- Light Clothing: Dress your baby in a single layer of light clothing. Avoid overdressing or heavy blankets.
- Lukewarm Bath: A lukewarm (not cold) sponge bath can help bring the temperature down.
- Fever-Reducing Medication: For babies over 3 months, you can use acetaminophen (Tylenol). For babies over 6 months, you can also use ibuprofen (Advil, Motrin). Always consult your doctor for the correct dosage based on your baby's weight.
A Note on Fevers & The App: Use MyBabyTracker to log your baby's temperature, medication doses, and symptoms. This creates a clear timeline to share with your pediatrician, helping them make a more accurate diagnosis.
Frequently Asked Questions
What rectal temperature is a fever for a baby?
A rectal temperature of 100.4°F (38°C) or higher is considered a fever in a baby. This is the most accurate way to measure temperature in an infant.
When is a baby's fever an emergency?
For babies under 3 months old, any fever of 100.4°F (38°C) or higher is considered an emergency and you should call your doctor immediately. For older babies, you should call the doctor for higher fevers (over 102°F or 38.9°C) or if the fever is accompanied by other signs of significant illness.
Can teething cause a high fever?
No. While teething may cause a slight elevation in temperature, it does not cause a true fever of 100.4°F (38°C) or higher. A high fever during teething usually indicates a separate, underlying illness — often an ear infection, which is easy to confuse with teething. Our guide to teething vs. ear infection covers how to tell them apart.
Should I give my baby medicine for a fever?
The main goal is to make your baby comfortable, not necessarily to eliminate the fever. For babies over 3 months who are uncomfortable, you can consider acetaminophen (Tylenol). For babies over 6 months, ibuprofen (Advil/Motrin) is also an option. Always consult your pediatrician for the correct dosage based on your baby's weight.
Common Myths About Fevers
- Myth: A high fever will cause brain damage.
- Fact: Fevers from infection do not cause brain damage. Only extreme temperatures (above 107.6°F / 42°C), typically caused by environmental factors like being in a hot car, are dangerous.
- Myth: Teething causes high fevers.
- Fact: Teething may cause a slight rise in temperature, but not a true fever over 100.4°F.
Remember, you know your baby best. If you are ever concerned, trust your instincts and call your pediatrician. A sick baby often also means disrupted sleep and feeds — logging both (see is baby getting enough milk?) gives your doctor a fuller picture. Not sure what's typical for your baby's stage when they're well? Our age-by-age baby guides cover normal sleep, feeding, and growth from newborn through 24 months. You can track temperatures, meds, and symptoms with a free MyBabyTracker account.
This article is for informational purposes only and is not a substitute for professional medical advice.