A temperature of 100.4°F (38°C) or higher, taken rectally, is a fever in a 4-month-old. At this age, a fever is usually the body fighting off a minor infection or responding to recent vaccinations, but it needs attention. Here’s what to do at home, what to watch for, and when to call your pediatrician.
Get an Accurate Temperature First
A rectal thermometer is the most reliable way to check a 4-month-old’s temperature. Forehead strips and armpit readings can be off by a degree or more, which matters when you’re deciding whether your baby truly has a fever. Use a digital rectal thermometer with a small amount of petroleum jelly on the tip, insert it gently about half an inch, and hold it in place until it beeps. If the reading is 100.4°F or above, your baby has a fever.
Write down the temperature and the time you took it. Your pediatrician will ask for this information if you call, and tracking the pattern helps you notice whether the fever is rising, falling, or holding steady.
Keep Your Baby Comfortable
Dress your baby in one layer of lightweight clothing. It’s tempting to bundle a feverish baby in blankets, especially if they seem to have chills, but extra layers trap heat and can push the fever higher. For sleep, one lightweight blanket is enough. Keep the room at a comfortable temperature. If the room feels warm or stuffy, a fan circulating air in the room (not pointed directly at your baby) can help.
A lukewarm sponge bath can bring some relief. Avoid cold water or rubbing alcohol, both of which can cause shivering and actually raise your baby’s core temperature. If your baby seems uncomfortable during the sponge bath, stop and try again later.
Watch for Dehydration
Fever makes babies lose fluids faster than usual, so hydration is your top priority. Continue breastfeeding or formula feeding on your normal schedule, and offer extra feeds if your baby is willing. Don’t give water to a 4-month-old. Breast milk or formula provides all the fluids they need.
Count wet diapers. A healthy 4-month-old should produce at least six wet diapers in 24 hours. Fewer than six is an early sign of dehydration. More serious signs include a sunken soft spot on the top of the head, no tears when crying, dry lips and mouth, and extreme sleepiness or irritability. If your baby is only producing one or two wet diapers a day, that signals severe dehydration and needs immediate medical attention.
Fever After 4-Month Vaccinations
Four-month-olds typically receive several vaccines at their well-child visit, and a low-grade fever in the first day or two afterward is common. The fever is a sign the immune system is responding to the vaccine, and it usually resolves on its own within 24 to 48 hours. You may also notice mild fussiness, redness at the injection site, or a slight rash.
A post-vaccination fever that climbs above 102°F, lasts longer than 48 hours, or comes with symptoms like persistent crying or difficulty breathing is worth a call to your pediatrician. These patterns fall outside the expected vaccine response.
Medication: What’s Safe at 4 Months
Acetaminophen (the active ingredient in Infant Tylenol) is the only fever reducer generally considered for babies this age, but it should not be given to children under 2 years old without your doctor’s guidance. If your pediatrician gives the go-ahead, they’ll tell you the exact dose based on your baby’s weight. The liquid form comes in a concentration of 160 mg per 5 mL, and doses are given every 4 hours as needed, with no more than 5 doses in 24 hours.
Ibuprofen (the active ingredient in Infant Advil or Motrin) is not safe for babies under 6 months old. It has not been found to be safe in that age group and is not FDA-approved for infants younger than 6 months. Don’t give it even if you have it on hand from an older child.
Never give aspirin to any child or teenager. It’s linked to a rare but serious condition that affects the liver and brain.
When to Call Your Pediatrician
At 4 months old, your baby is past the newborn danger zone (under 3 months, any fever is treated as an emergency), but fevers still deserve a lower threshold for calling the doctor than they would in an older child. Call your pediatrician if:
- The fever lasts more than 24 hours. For children under 2, a fever persisting beyond a full day warrants a call even if your baby seems otherwise okay.
- The temperature reaches 102°F (38.9°C) or higher. Higher fevers in young infants need professional assessment.
- Your baby is unusually sleepy or hard to wake. Some drowsiness with a fever is normal, but a baby who won’t rouse for feeding or seems limp is a different situation.
- Your baby refuses to eat. Skipping one feed isn’t alarming, but consistently refusing breast milk or formula raises the risk of dehydration.
- You notice signs of dehydration. Fewer than six wet diapers in 24 hours, a sunken soft spot, or no tears when crying.
- A rash appears. Some rashes with fever are harmless, but others signal infections that need treatment.
When to Go to the Emergency Room
Some symptoms alongside a fever require immediate care, not a phone call. Head to the ER or call 911 if your baby has a seizure. Febrile seizures (seizures triggered by fever) can happen in young children and are terrifying to witness, though most are brief and don’t cause lasting harm. Your baby still needs to be evaluated right away.
Other reasons to go immediately: difficulty breathing, a bluish tint to the lips or skin, inconsolable crying that lasts for hours, or your baby becoming completely unresponsive. Trust your instincts. If something feels seriously wrong, it’s always better to have your baby checked and sent home than to wait.