Written & Medically Reviewed by Dr. G
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Board-Certified Pediatrician
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35+ Years of Experience
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About Dr. G
💡 Quick Answer
If your baby is under 3 months old and has a temperature of 100.4°F (38°C) or higher, go to the emergency department.
Do not wait for the fever to get higher. Even a baby who looks well needs to be checked.
✓ Key Takeaways
- 100.4°F (38°C) or higher is a fever in a young baby.
- A baby under 3 months with a fever needs medical care right away.
- A rectal temperature gives the best reading at this age.
- Do not wait for the fever to reach 102°F or 103°F.
- Do not give fever medicine first unless a doctor tells you to.
- A baby may look well and still need to be checked.
Why Is Fever Different in a Young Baby?
Young babies can get sick quickly.
Their immune system is still developing. They may also show very few signs of illness.
Sometimes fever is the only clear sign that something is wrong.
Most fevers are caused by viruses. But doctors also need to rule out serious bacterial infections.
💜 Dr. G's Pearl
With a young baby, age matters as much as the temperature.
A temperature that may be watched at home in an older child is treated very differently in a newborn.
What Temperature Is a Fever?
For a baby under 3 months, 100.4°F (38°C) or higher is a fever.
Parents sometimes hear the words “low-grade fever.” Do not let that phrase cause you to wait.
If a 2-month-old reaches 100.4°F, the baby needs to be checked.
How Should I Take My Baby's Temperature?
A digital rectal thermometer gives the most accurate reading for a baby under 3 months.
Follow the thermometer directions carefully. Be gentle.
If you get a reading of 100.4°F (38°C) or higher, your baby needs medical care.
🚨 Go to the Emergency Department
A baby under 3 months with a temperature of 100.4°F (38°C) or higher should be seen right away.
Do not stay home to see if the fever goes away.
What If My Baby Looks Fine?
That is reassuring, but it does not change the advice.
Young babies can look well early in an illness.
They still need to be checked when they have a true fever.
What Can Cause Fever in a Baby?
Many fevers are caused by common viral infections.
Doctors also need to think about bacterial infections. These can include:
- A urinary tract infection
- A bloodstream infection
- Pneumonia
- Meningitis
Parents cannot tell the cause from the temperature alone. That is why the baby needs an exam.
What Will Happen at the Hospital?
The medical team will examine your baby.
They will ask about feeding, wet diapers, breathing, sleepiness, and other symptoms.
Your baby's exact age matters.
Some babies need urine or blood tests. Some need other tests, medicine, or time in the hospital.
The medical team will decide what is needed after seeing your baby.
Should I Give Fever Medicine First?
Usually, no.
Do not give acetaminophen to a baby under 3 months for a new fever unless a healthcare professional tells you to.
The first step is getting your baby checked.
Ibuprofen should not be used in babies younger than 6 months unless a doctor tells you to use it.
🏠 What Parents Should Do
- Write down the temperature.
- Write down how you took it.
- Note the time.
- Notice how your baby is feeding and breathing.
- Think about the number of wet diapers.
- Go for medical care right away.
What If the Fever Started After Vaccines?
Vaccines can sometimes cause fever.
But a baby under 3 months who reaches 100.4°F (38°C) still needs medical advice and evaluation.
Tell the doctor when the vaccines were given and when the fever began.
What Warning Signs Are Even More Urgent?
Any fever in a baby under 3 months needs care. Some signs make the situation even more urgent.
Watch for:
- Trouble breathing
- Blue, gray, or very pale skin or lips
- A baby who is hard to wake
- Poor feeding
- Very few wet diapers
- Repeated vomiting
- A seizure
- A purple or bruise-like rash
- A baby who looks very sick
🚑 When to Call 911
Call 911 for severe trouble breathing, blue or gray color, a seizure, unresponsiveness, or another life-threatening problem.
Frequently Asked Questions
Is 100.4°F really a fever in a newborn?
Yes. A temperature of 100.4°F (38°C) or higher is a fever in a young baby.
What about a low-grade fever in a 2-month-old?
If your 2-month-old reaches 100.4°F (38°C), seek medical care. Do not wait for the temperature to rise.
What if my baby is eating normally?
That is a good sign, but a baby under 3 months with a true fever still needs to be checked.
What if the fever goes away?
The earlier fever still matters. Tell the doctor the highest temperature you saw and how you measured it.
Can I watch the fever at home?
Not if your baby is under 3 months and reached 100.4°F (38°C) or higher. Your baby needs medical care.
💜 Dr. G's Pearl
Do not wait for a young baby's fever to become “high.”
For a baby under 3 months, 100.4°F (38°C) is the number that matters.
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References & Medical Sources
⚕️ Medical Disclaimer
This article is for general education and does not replace medical care. A baby under 3 months with a temperature of 100.4°F (38°C) or higher needs prompt medical evaluation. Call 911 for a life-threatening emergency.
