A rectal temperature of 100.4°F (38°C) or higher in a newborn under 3 months old is a medical emergency. Unlike older children, newborns cannot reliably fight infections on their own, and a fever at this age can signal a serious illness that needs immediate evaluation. If your newborn has a fever, call your pediatrician right away or go directly to the emergency room.
How to Take Your Newborn’s Temperature
A rectal thermometer is the most accurate method for babies under 3 months. Armpit readings can serve as a quick first check, but they’re the least reliable and your doctor will likely want a rectal confirmation. Ear thermometers are not recommended for newborns either, as the ear canal is too small for an accurate reading.
To take a rectal temperature, apply a small amount of petroleum jelly to the tip of a digital thermometer and gently insert it about half an inch into the rectum. Hold it in place until the thermometer beeps. If the reading is 100.4°F or above, that confirms a fever.
Why Newborn Fevers Are Treated So Urgently
A newborn’s immune system is immature. Infections that an older child could fight off on their own, like urinary tract infections or bloodstream infections, can escalate quickly in the first few months of life. Doctors treat any fever in this age group as potentially serious until testing proves otherwise. This is not an overreaction. It’s the standard approach at every children’s hospital.
Signs That Require 911
Most newborn fevers warrant a call to your pediatrician or an ER visit. But certain symptoms alongside a fever mean you should call 911 immediately:
- Your baby is not moving or is limp
- You cannot wake your baby
- Purple or blood-colored spots or dots appear on the skin
Other signs that mean you should head to the ER without delay include a swollen soft spot on top of the head, difficulty waking the baby, or your baby seeming “out of it” and not alert when awake. Trust your instincts here. If something looks wrong, act on it.
What Happens at the Hospital
When you bring a febrile newborn to the emergency department, doctors will run tests to find the source of the infection. This can feel overwhelming, but knowing what to expect helps.
For the youngest babies (under 3 weeks old), the workup is the most thorough. Doctors will typically draw blood for a blood culture, collect a urine sample, and perform a lumbar puncture (spinal tap) to check the spinal fluid for signs of meningitis. They’ll also test for herpes simplex virus, which can cause serious illness in newborns. For babies between 3 and 8 weeks old, the process is similar, though the spinal tap may not always be needed depending on how your baby looks clinically and what the initial blood and urine results show.
These tests check for bacterial infections that need antibiotic treatment. Your baby will likely be started on antibiotics while the cultures are processed, which can take 24 to 48 hours. If all the results come back negative, antibiotics are stopped and you go home. If something is found, your baby stays for targeted treatment. Many newborn fevers turn out to be caused by common viruses, but doctors can’t distinguish a harmless virus from a dangerous bacterial infection without testing.
What You Can Do at Home While Waiting
While you’re waiting to reach your pediatrician or preparing to head to the ER, there are a few things you can safely do to keep your baby comfortable.
Dress your baby in one layer of lightweight clothing. Do not bundle them in extra blankets or layers, even if they seem to have chills. Extra layers trap heat and can push the fever higher. For sleep, one lightweight blanket is enough. If the room feels warm or stuffy, a fan can help circulate air.
A lukewarm sponge bath can help bring the temperature down slightly. Never use cold water, ice, or alcohol rubs. These cause shivering, which actually raises the body’s core temperature and makes things worse.
Keep feeding your baby. Fevers increase fluid loss, and the biggest risk outside of infection itself is dehydration. Whether you’re breastfeeding or formula feeding, offer feeds more frequently than usual. After the first week of life, a well-hydrated baby produces at least 6 wet diapers per day, with no more than 8 hours between wet diapers. If you’re noticing fewer wet diapers than that, mention it to your doctor.
Do Not Give Fever-Reducing Medication
This is one of the most important things to know: do not give your newborn acetaminophen (Tylenol) or ibuprofen (Advil, Motrin) without explicit instructions from a doctor. Ibuprofen is not considered safe for any baby under 6 months old. Acetaminophen is sometimes used in infants over 3 months at doctor-directed doses, but in a newborn, giving medication before the baby is evaluated can mask symptoms and delay diagnosis. Let the medical team decide what’s appropriate after they’ve examined your child.
Fever in Older Infants (3 to 12 Months)
Once your baby passes the 3-month mark, the urgency around fevers changes. A fever in a 4-month-old still warrants a call to your pediatrician, but it doesn’t automatically require an ER visit and a full workup the way it does in the newborn period. For children under 2 years, call your doctor if the fever lasts more than 24 hours, even if your baby seems otherwise fine. For children 2 and older, the threshold extends to 72 hours.
The key difference is that older infants have a more developed immune system and are better equipped to handle common viral infections. Your pediatrician can guide you on whether a visit is needed based on your baby’s age, temperature, symptoms, and overall behavior.