A fever in a newborn under 2 months old with a rectal temperature above 100.4°F (38°C) requires an immediate trip to the emergency department, no matter how well the baby seems. For older infants and children, the decision depends less on the number on the thermometer and more on how your child is acting, how long the fever has lasted, and whether other worrying symptoms are present.
The Newborn Rule: Under 2 Months
This is the clearest guideline in pediatric fever care. If your baby is younger than 2 months and has a rectal temperature above 100.4°F (38°C), go to an emergency department right away. Young infants have immature immune systems, and a fever at this age can signal a serious bacterial infection that progresses quickly. Don’t wait to see if it resolves, and don’t give fever-reducing medicine first.
Rectal thermometers are recommended for children under 3 years because they give the most accurate core temperature reading. Forehead and ear thermometers can be convenient for older kids, but for a baby this young, a rectal reading is what doctors rely on to make decisions.
Age-Based Temperature Thresholds
Fever is broadly defined as a rectal temperature of 100.4°F (38°C) or higher. But the temperature that should prompt a call to your pediatrician shifts with age:
- Under 2 months: Any rectal temperature above 100.4°F. Go to the ER.
- 3 to 6 months: A temperature of 101°F (38.3°C) or higher warrants a call to your pediatrician. At 102.2°F (39°C) or above, guidelines from NICE classify these babies as being at intermediate risk for serious illness.
- 6 months to 2 years: Call your pediatrician if a fever of 100.4°F or higher lasts longer than 24 hours, even if your child seems otherwise fine.
- Over 2 years: A fever lasting more than 72 hours (3 days) is the threshold for contacting your pediatrician.
These are starting points, not rigid cutoffs. A child with a 101°F fever who is limp and unresponsive needs care far more urgently than a child with a 103°F fever who is drinking fluids and playing between naps.
Behavior Matters More Than the Number
The American Academy of Pediatrics emphasizes that how your child acts is a better indicator of severity than the thermometer reading alone. A good sign: your child perks up, makes eye contact, and plays or interacts with you after receiving a dose of fever-reducing medicine. That suggests the underlying cause is likely a routine virus the body is fighting off normally.
Call your pediatrician right away if your child still “acts sick” once the fever comes down. This means they remain drowsy, uninterested in their surroundings, or unusually fussy even after the medicine takes effect. A child who looks very ill, won’t engage with you, or is difficult to wake up needs prompt evaluation regardless of what the thermometer says.
Signs That Need Emergency Care
Certain symptoms alongside a fever mean you should head to the ER or call 911, not wait for a callback from your pediatrician’s office:
- Difficulty breathing: Fast, labored, or shallow breaths. Skin pulling inward between the ribs with each breath. Blue-tinged lips or face.
- Extreme lethargy: Unusually sleepy, hard to wake, or not responding to your voice or touch.
- Stiff neck: Your child resists moving their neck or can’t bend it forward. This can be a sign of meningitis.
- Rash that doesn’t fade: Press a glass against the rash. If the spots don’t blanch (turn white), or if you see purple spots on the skin, this could indicate a serious bacterial infection.
- Bulging soft spot: In babies, a tense or bulging fontanelle (the soft spot on top of the head) may signal increased pressure inside the skull.
- Persistent, unusual crying: A high-pitched cry that sounds different from normal fussiness and can’t be soothed.
Watching for Dehydration
Fever increases fluid loss, and young children can become dehydrated faster than adults. This is especially concerning when a child is also vomiting or refusing to drink. Keep track of how much your child is taking in and how much is coming out.
Signs of dehydration to watch for: fewer than six wet diapers in 24 hours, no tears when crying, a dry mouth or cracked lips, and sunken eyes. In babies, a sunken soft spot on top of the head is another warning sign. If you notice several of these together, your child needs medical attention. Severe dehydration sometimes requires IV fluids, so catching it early by pushing small, frequent sips of fluid can make a real difference.
Febrile Seizures
Febrile seizures affect 2% to 5% of young children, with the peak risk between 12 and 18 months. They typically involve sudden shaking or stiffening of the body and are triggered by a rapid rise in temperature, not necessarily a high fever. Watching your child have a seizure is terrifying, but most febrile seizures are classified as “simple,” meaning they last under 15 minutes and don’t recur within 24 hours. Simple febrile seizures are considered benign and don’t cause brain damage or increase the risk of epilepsy.
That said, any first seizure warrants a call to your pediatrician. If a seizure lasts longer than 5 minutes, involves only one side of the body, or happens more than once in 24 hours, call 911. These features make it a “complex” febrile seizure, which requires a more thorough evaluation. About 30% of children who have one febrile seizure will have another during early childhood, so your pediatrician can help you prepare a plan for future episodes.
Post-Vaccination Fevers
A low-grade fever after routine childhood immunizations is normal and not a reason to call your pediatrician. These fevers typically start within 12 hours of the shot and resolve within 1 to 2 days. For temperatures between 100°F and 102°F (37.8 to 39°C) after a vaccine, fever-reducing medicine isn’t usually needed unless your child seems uncomfortable.
Contact your pediatrician within 24 hours if a post-vaccine fever lasts longer than 3 days. At that point, the fever is less likely related to the vaccine and more likely caused by something else that started around the same time.
Recurring Fevers Without a Clear Cause
Some children develop a pattern of fevers that come and go over weeks or months. If your child has three or more episodes of unexplained fever within a 6-month period, with at least 7 days between episodes, doctors consider this a recurrent fever pattern that deserves investigation. Between fever episodes, these children often feel completely well.
Your pediatrician will likely ask detailed questions about the timing, duration, and accompanying symptoms of each episode. If your child also has weight loss, night sweats, repeated infections with unusual germs, or recent travel to areas with endemic infections, further testing or a referral to a specialist is typically the next step. In many cases, recurrent fevers in children turn out to have a benign explanation, but the pattern itself is worth documenting and discussing.