A 4-month-old has a fever when their temperature reaches 100.4°F (38°C) or higher. If your baby hits that number and seems unwell, call your pediatrician. At this age, fevers always warrant a phone call to your doctor’s office, but most causes are minor infections or reactions to vaccines, and you can manage many fevers safely at home while you wait for guidance.
How to Take Your Baby’s Temperature
For babies under 3 years old, a rectal thermometer gives the most accurate reading. Forehead (temporal artery) thermometers are a close second and cause less discomfort, making them a reasonable alternative for a quick check. Ear thermometers aren’t considered accurate until after 6 months of age, so skip those for now. Armpit readings can work as a first pass, but they’re the least reliable method and your doctor may ask you to confirm with a rectal reading.
Forehead strips and pacifier thermometers that change color aren’t reliable enough to trust. Stick with a standard digital thermometer.
When to Call Your Pediatrician
For babies between 3 and 6 months old, the threshold for calling is any temperature of 100.4°F or higher. You should also call if your baby’s temperature is below that number but they seem noticeably unwell, unusually fussy, or harder to wake than normal.
Call right away if you notice any of these:
- Unusual sleepiness. Your baby is sleeping much more than normal or is difficult to wake up.
- Floppiness or weak muscle tone. Your baby feels limp when you pick them up.
- Signs of dehydration. Fewer wet diapers than usual, a dry mouth, crying with few or no tears, or a sunken soft spot on the top of the head.
- Persistent crying. Your baby is crying more than usual and very difficult to comfort.
- Fever lasting more than three days. Even a low-grade fever that doesn’t resolve deserves a call.
Trust your instincts. If something feels off about the way your baby looks or acts, that’s a valid reason to pick up the phone.
Fever After 4-Month Vaccinations
The 4-month well visit typically includes several vaccines, and a mild fever afterward is one of the most common reactions. Your baby might also be fussier than usual or have some redness at the injection site. These reactions are normal and usually resolve within a day or two. A lukewarm sponge bath and extra comfort nursing or bottle feeding can help. If you’re unsure whether your baby’s fever is from vaccines or something else, call your pediatrician to sort it out.
Medication for a 4-Month-Old
Acetaminophen (Tylenol) is the only fever-reducing medication considered appropriate at 4 months. Dosing is based on your baby’s weight, not their age. For infants weighing 6 to 11 pounds, the typical dose of infant liquid acetaminophen (160 mg per 5 mL) is 1.25 mL. For babies weighing 12 to 17 pounds, it’s 2.5 mL. You can repeat every 4 to 6 hours as needed, but no more than 5 times in a 24-hour period. Always confirm the dose with your pediatrician before giving it, especially the first time.
Ibuprofen (Advil, Motrin) is not recommended for babies under 6 months old. It carries risks to the kidneys and digestive system that make it unsafe at this age. Don’t give it even if you have infant ibuprofen at home from an older child. And never give aspirin to any infant or child.
Keeping Your Baby Comfortable at Home
Your goal isn’t necessarily to eliminate the fever entirely. Fever is part of the body’s immune response. The real priority is keeping your baby comfortable and well-hydrated.
Dress your baby in one layer of lightweight clothing. For sleep, a single lightweight blanket is enough. Resist the urge to bundle them up, even if they seem to have chills. Extra layers can trap heat and push the fever higher. Keep the room at a comfortable temperature. If it feels stuffy, a fan circulating air in the room is fine.
A lukewarm sponge bath can help bring a fever down, especially when combined with acetaminophen. Don’t use cold water, ice packs, or rubbing alcohol. Cold makes babies shiver, which actually raises their core temperature and makes things worse.
Offer frequent feedings. Whether you’re breastfeeding or formula feeding, shorter and more frequent sessions help maintain hydration. A baby with a fever loses fluid faster than usual through sweating and rapid breathing. Watch for wet diapers as your simplest hydration check. If you’re seeing noticeably fewer wet diapers than a normal day, that’s a sign to call your doctor.
Febrile Seizures
Febrile seizures are convulsions triggered by fever, and while they’re frightening to witness, they’re almost always harmless. Children between 6 months and 5 years are most at risk, with the peak between ages 1 and 3. At 4 months, your baby is slightly below the most common age range, but it’s still worth knowing what to look for.
During a febrile seizure, a baby may lose consciousness, shake or stiffen in the arms and legs, roll their eyes, or lose control of their bladder or bowels. The most common type lasts a few seconds to 15 minutes and happens only once in a 24-hour period. If your baby has a seizure, lay them on a firm surface on their side, don’t put anything in their mouth, and call 911. Even though most febrile seizures end on their own and don’t cause lasting harm, a first seizure in an infant this young should always be evaluated immediately.
Children who have a febrile seizure at a younger age, who have a family history of febrile seizures, or whose seizure was the first sign of illness (before you even noticed the fever) are more likely to have another one in the future. Your pediatrician can help you understand your baby’s individual risk.