Your baby has a fever when their body temperature reaches 100.4 degrees Fahrenheit or higher. Fevers usually signal an infection with a bacteria or virus, although there are other possible causes. Every baby will run a fever at some point, so it’s important to know how to correctly take your baby’s temperature at different ages.
(Note that a fever in a newborn baby under 3 months of age should trigger an immediate call to your doctor. Fevers in babies this age can mean that something more serious is going on, such as an infection in the blood, urine, or central nervous system.)
The American Academy of Pediatrics urges parents to avoid mercury thermometers because of the risk of mercury poisoning if the thermometer breaks. Instead, choose a digital thermometer. There are several types of digital thermometers:
- Oral—These are placed into the baby’s mouth and held there until it beeps. Oral measurement is not recommended for newborns and small infants.
- Armpit—This type is tucked under a person’s armpit and held in place until it beeps.
- Rectal digital—These thermometers are gently inserted into a baby’s anus just past the thermometer’s metal tip. The reading is reliable and fast, which makes it ideal for newborns and small infants.
- Tympanic, or ear, thermometers—These are often a bit more expensive and are commonly used in the doctor’s office. They require more skill to use correctly but can be very accurate.
- Temporal lobe thermometer—These are placed near baby’s temple to read the body temperature. Like tympanic thermometers, they are a bit more expensive and can require practice.
Until your baby is six months old, rectal temperature taking is the most accurate. To correctly take a rectal temperature, follow these steps:
- Try to calm your baby with soothing and rocking. It can be hard to get an accurate reading from a kicking and wiggling baby.
- Position your baby belly down, either on your lap or on a firm surface.
- Use a lubricant like Vaseline and gently slide the thermometer into baby’s anus about one inch. A rectal thermometer should never be forced or cause any signs of pain.
- Wait until the thermometer beeps before removing it.
- After use, wash with warm soapy water and let dry completely.
At six months of age, the ear or temporal thermometers may be used, but this can be a difficult method to get an accurate reading. Ear thermometers tend to be more expensive and require accurate placement to get a good reading. Carefully follow the manufacturer’s guidelines. If in doubt, take a rectal temperature.
Reviewed by Dr. Sara Connolly, January 2020
- Newborns that have a fever above 100.4°F should see a doctor immediately.
- Do not use a mercury thermometer.
- Rectal temperature taking is the most accurate until six months of age.
- After six months, you can opt for using a thermometer in the ear, armpit, or temporal region, although they take some practice and are not accurate unless used correctly.