Fever in a Newborn: Why 100.4°F Means Call Right Away

Key takeaways: For babies under 3 months, a rectal temperature of 100.4°F (38°C) or higher needs prompt medical evaluation, day or night. Don’t give fever medicine first. Rectal is the most accurate method. A low temperature with poor feeding is also a warning sign.

In older children, a fever is often just a sign the body is fighting a cold. In young babies, the rules are different. A fever in a baby younger than 3 months can be the only early sign of a serious infection, and young infants can get sick quickly. That’s why pediatricians ask parents to call right away.

What Counts as a Fever?

For babies under 3 months old, a fever is a rectal temperature of 100.4°F (38.0°C) or higher. It doesn’t matter if your baby otherwise seems well. Call your baby’s doctor or after-hours line right away, or go to the emergency department.

Why Young Babies Are Different

Newborns’ immune systems are still immature. In the first weeks of life, babies with fever have a meaningful chance of a serious bacterial infection, such as:

• A urinary tract infection (the most common).
• A bloodstream infection (bacteremia).
• Meningitis, an infection of the lining of the brain and spinal cord.

Young babies with these infections may not look very sick at first. They often don’t show the specific symptoms older kids do, so testing is the only reliable way to find out.

What Happens When You Call

The AAP’s 2021 guideline for well-appearing infants 8 to 60 days old with fever helps doctors decide on testing based on age and lab results. Depending on your baby’s age and exam, your baby may have:

• A urine test.
• Blood tests, including a blood culture and markers of inflammation.
• In some cases, especially in the first month of life, a spinal tap (lumbar puncture) to check for meningitis.
• Antibiotics while waiting for results, and sometimes a hospital stay.

Babies under about 1 month old are most likely to need the full workup. This can feel like a lot, but it’s the safest approach for this age.

How to Take Your Baby’s Temperature

For babies under 3 months, rectal temperature is the most accurate. Forehead (temporal artery), ear, and armpit readings are less reliable in young infants. If one of those methods shows a fever, confirm with a rectal temperature or call your doctor.

1. Use a digital thermometer. Label it “rectal” and use it only for that.
2. Clean the tip with soap and water or rubbing alcohol, then rinse.
3. Put a small amount of petroleum jelly on the tip.
4. Place your baby on their back with knees bent toward the chest, or tummy-down across your lap.
5. Gently insert the tip about half an inch to one inch into the bottom. Stop if you feel resistance.
6. Hold the thermometer and your baby still until it beeps.
7. Write down the temperature and the time.

Don’t Give Medicine First

Don’t give acetaminophen (Tylenol) to a baby under 12 weeks without talking to a medical professional first. Fever medicine can lower the temperature and hide important information your doctor needs. Never give ibuprofen to babies under 6 months, and never give aspirin to children.

Can Bundling Cause a Fever?

Babies who are overdressed or in a very warm room can have a slightly higher temperature. If you think this might be the reason, remove extra layers, wait 15 to 30 minutes, and recheck. If the rectal temperature is still 100.4°F or higher, call. When in doubt, call.

Low Temperature Is a Warning Sign Too

In young babies, a temperature that’s too low can also signal infection. A rectal temperature below about 97.5°F (36.4°C) that doesn’t improve with skin-to-skin or an extra layer, especially with poor feeding or unusual sleepiness, needs a call.

Other Warning Signs in Young Babies

Call your baby’s doctor promptly if your baby:

• Is feeding poorly or refuses several feeds.
• Is unusually sleepy, floppy, or hard to wake.
• Is unusually irritable or inconsolable.
• Has fewer wet diapers than usual.
• Is vomiting forcefully or repeatedly, or vomit is green.
• Has a rash of tiny red or purple spots that don’t fade when pressed.

Call 911 if your baby: is struggling to breathe, has blue or gray lips or face, has a pause in breathing of 20 seconds or longer, can’t be woken, or has a seizure.

Preventing Infections in the First Months

• Wash hands before holding or feeding your baby, and ask others to do the same.
• Keep sick people away from your baby, and limit crowded indoor places in the first months.
• Make sure caregivers are up to date on whooping cough (Tdap), flu, and other recommended vaccines.
• Keep your baby’s environment smoke-free.
• Breastfeed if you can; breast milk provides antibodies.

The Bottom Line

For a baby younger than 3 months, a rectal temperature of 100.4°F or higher means call right away, even at 3 a.m., even if your baby seems fine. Have a digital thermometer ready before you need it.

For more on newborn health and when to call the doctor, see my book From First Breath to First Birthday.

Sources

1. Pantell RH, Roberts KB, Adams WG, et al; Subcommittee on Febrile Infants. Evaluation and management of well-appearing febrile infants 8 to 60 days old. Pediatrics. 2021;148(2):e2021052228.
2. Sullivan JE, Farrar HC; Section on Clinical Pharmacology and Therapeutics; Committee on Drugs. Fever and antipyretic use in children. Pediatrics. 2011;127(3):580-587.
3. Biondi EA, Lee B, Ralston SL, et al. Prevalence of bacteremia and bacterial meningitis in febrile neonates and infants in the second month of life: a systematic review and meta-analysis. JAMA Netw Open. 2019;2(3):e190874.
4. Kuppermann N, Dayan PS, Levine DA, et al. A clinical prediction rule to identify febrile infants 60 days and younger at low risk for serious bacterial infections. JAMA Pediatr. 2019;173(4):342-351.
5. American Academy of Pediatrics. How to take a child’s temperature. HealthyChildren.org. https://www.healthychildren.org
6. World Health Organization. WHO Recommendations on Maternal and Newborn Care for a Positive Postnatal Experience. Geneva: WHO; 2022.

This article is for general education and is not a substitute for individualized medical advice. Always talk with your baby’s health care provider or your own provider about your specific situation. If you think your baby or you may have a medical emergency, call 911.