A fever in a baby is a rectal temperature of 100.4°F (38°C) or higher. That single number is the standard threshold used by pediatricians regardless of your baby’s age, but what you should do about it depends heavily on how old your baby is. A fever in a one-week-old is treated as a medical emergency, while the same temperature in a nine-month-old is usually manageable at home.
The 100.4°F Threshold
Normal body temperature hovers around 98.6°F (37°C), though it fluctuates throughout the day. A reading of 100.4°F or above, taken rectally, is the definition of fever in infants. Anything below that, even if it feels warm to you, is not technically a fever.
This matters because parents sometimes confuse a slightly warm forehead with a true fever. Teething, for example, can nudge your baby’s temperature up a bit, but it does not cause a real fever. If your baby’s temperature hits 100.4°F or higher, that signals an infection, not teething, and you should treat it accordingly.
Why Age Changes Everything
For babies under 8 weeks old, a fever of 100.4°F or higher is considered urgent. Their immune systems are too immature to reliably fight off serious bacterial infections, and a fever may be the only visible sign that something dangerous is happening. The American Academy of Pediatrics has specific evaluation guidelines broken into narrow age windows (8 to 21 days, 22 to 28 days, and 29 to 60 days) because even a few weeks of age changes the risk profile significantly. If your newborn has a fever in this range, go to the emergency room without waiting.
For babies 3 to 6 months old, a fever of 102°F or higher, or any fever that lasts more than a day, warrants a call to your pediatrician. At this age, serious infections are less likely than in a newborn but still possible.
For babies older than 6 months, fever is more common and usually less concerning on its own. How your baby is acting matters more than the number on the thermometer. A baby with a 101°F fever who is drinking, making eye contact, and playing between naps is in a very different situation than a baby with the same fever who is limp, inconsolable, or refusing to eat.
How to Take an Accurate Temperature
Rectal thermometers are the gold standard for babies. Other methods, including armpit, forehead, and ear readings, are less reliable. Armpit temperatures are the least accurate of all common methods. Ear thermometers can be thrown off by earwax or the shape of a small ear canal. If you use any method other than rectal and the reading seems off, follow up with a rectal reading to be sure.
To take a rectal temperature, apply a small amount of petroleum jelly to the tip of a digital thermometer and insert it about half an inch. Hold it in place until it beeps. The whole process takes under a minute, and while babies don’t love it, it gives you the most trustworthy number.
Signs Your Baby Needs Immediate Attention
Beyond the temperature itself, certain signs suggest your baby needs emergency evaluation. Babies who appear ill, meaning they look pale or bluish, are unusually floppy or difficult to wake, breathe rapidly, or won’t stop crying despite comfort, need to be seen right away regardless of what the thermometer says. Premature babies (born before 37 weeks), babies with complex medical histories, or those who spent time in intensive care after birth may carry a higher risk of serious infection and deserve a lower threshold for seeking help.
Watching for Dehydration
Fever increases fluid loss, and babies dehydrate faster than adults. Count wet diapers: fewer than usual is an early warning sign. Other signs of dehydration include a sunken soft spot on top of the head, sunken eyes, few or no tears when crying, and unusual drowsiness or irritability. If you notice any of these, contact your pediatrician or seek urgent care. Offer frequent breastfeeds or formula to keep fluids going during a fever.
Febrile Seizures
About 3 to 4 out of every 100 children will experience a febrile seizure, a convulsion triggered by fever. During one, your baby may stiffen, twitch, roll their eyes, become unresponsive, or have changes in breathing or skin color. Most febrile seizures last less than one to two minutes and, while terrifying to watch, do not cause brain damage or long-term problems. If it happens, lay your baby on their side on a flat surface, don’t put anything in their mouth, and time the seizure. If it lasts longer than five minutes, call emergency services.
Treating a Fever at Home
For babies under 3 months, do not give any fever-reducing medication without a doctor’s guidance. This age group needs medical evaluation, not home treatment.
Acetaminophen (Tylenol) can be given to babies 3 months and older, dosed by weight rather than age. The standard infant liquid contains 160 mg per 5 mL. A baby weighing 12 to 17 pounds gets 2.5 mL, while one weighing 18 to 23 pounds gets 3.75 mL. Always use the syringe or measuring device that comes with the product, not a kitchen spoon.
Ibuprofen (Advil, Motrin) should not be given to babies younger than 6 months. After 6 months, it’s dosed by weight as well. For a baby weighing 18 to 23 pounds, the typical dose is 3.75 mL of the liquid suspension (100 mg per 5 mL). Check the concentration on the label carefully, as infant drops and children’s liquid contain different amounts per milliliter.
Beyond medication, keep your baby in light clothing, offer extra fluids, and keep the room comfortable. Lukewarm sponge baths can help but aren’t necessary. Avoid ice baths or rubbing alcohol, which can be dangerous.
What a Fever Actually Does
Fever itself isn’t the enemy. It’s your baby’s immune system raising body temperature to fight off viruses and bacteria. Most fevers in babies over 3 months are caused by common viral infections and resolve within a few days. The goal of treatment isn’t to eliminate the fever entirely but to keep your baby comfortable enough to rest and drink. A baby who is sleeping well and staying hydrated with a temperature of 101°F doesn’t necessarily need medication at all.