A fever in a newborn is a rectal temperature of 100.4°F (38°C) or higher. This threshold applies to all babies under 3 months old, and any reading at or above it warrants an immediate call to your pediatrician. Unlike fevers in older children or adults, a newborn fever is never a “wait and see” situation.
Why 100.4°F Is the Cutoff
The 100.4°F threshold isn’t arbitrary. The American Academy of Pediatrics uses this number in its clinical practice guidelines for evaluating infants between 8 and 60 days old. At this age, a baby’s immune system is still immature, which means infections can escalate quickly and produce fewer obvious warning signs than they would in an older child. A fever may be the only early clue that something serious is developing.
The risk increases with higher temperatures. For every degree Celsius above 38.5°C (about 101.3°F), the odds of a serious bacterial infection nearly double. Babies under 28 days old face the highest overall risk, and the AAP treats this youngest group with the most caution, but any infant under 3 months with a fever of 100.4°F or above needs medical evaluation.
What Makes Newborn Fevers Dangerous
In older kids, a fever usually means a common virus that will pass on its own. In newborns, the same temperature reading can signal infections that move fast, including bacterial infections of the blood or spinal fluid. Herpes simplex virus (HSV) is another concern: it affects roughly 1,500 to 2,000 newborns in the U.S. each year, and babies in the second week of life (7 to 14 days old) are at the highest risk. Left untreated, neonatal HSV has a mortality rate as high as 75%.
This is why pediatricians take newborn fevers so seriously. Even a baby who looks perfectly fine can have an infection brewing beneath the surface. The evaluation for a febrile newborn typically involves blood work, a urine sample, and sometimes a spinal fluid test to rule out these possibilities.
Rectal Temperature Is the Standard
For babies under 3 months, a rectal thermometer is the most accurate way to measure body temperature. Forehead and underarm readings can be convenient for older children, but they aren’t reliable enough in newborns to make medical decisions. If you get a concerning reading from a forehead or underarm thermometer, follow up with a rectal measurement before calling your pediatrician, so you can give them an accurate number.
To take a rectal temperature safely, apply a small amount of petroleum jelly to the tip of a digital thermometer. Lay your baby face-down on your lap or face-up on a firm surface, and gently insert the tip about half an inch into the rectum. Hold the thermometer in place until it beeps. Clean it thoroughly with rubbing alcohol or soap and water afterward, and label it so it’s only used rectally.
Signs to Watch Beyond Temperature
Temperature is the most objective sign, but your baby’s behavior matters too. Contact your pediatrician right away if you notice any of these alongside a fever, or even without one:
- Feeding changes: missing two or more feedings in a row, or eating poorly
- Unusual sleepiness: sleeping more than normal, being hard to wake up, or seeming limp or floppy
- Excessive crying: fussiness that won’t stop or keeps getting worse
- Dehydration signs: fewer wet diapers, dry mouth, crying with fewer tears, or a sunken soft spot on the head
- Breathing trouble: fast or labored breathing, or skin and lips that look blue, purple, or gray
- Vomiting: throwing up after feedings or not keeping liquids down for eight hours
- Skin changes: a rash that appears quickly or blisters, or redness and oozing around the belly button or circumcision site
A baby who is not conscious, seems withdrawn, or acts strangely needs emergency care immediately, whether or not a fever is present.
Don’t Give Fever Medicine Without Guidance
Your instinct might be to bring the temperature down with infant acetaminophen (Tylenol), but fever-reducing medication is not recommended for babies under 2 years old without a doctor’s direction. Ibuprofen (Motrin, Advil) is off-limits entirely for babies under 6 months. For a newborn specifically, giving medication at home can mask the fever and delay the evaluation your baby actually needs.
In the first 12 weeks of life, a fever is a signal that your baby needs to be seen and tested in a medical setting. The priority isn’t lowering the number on the thermometer. It’s finding and treating whatever is causing it. If your newborn’s rectal temperature hits 100.4°F, skip the medicine cabinet and call your pediatrician right away, or head to the emergency room if it’s after hours.