Taking a rectal temperature is the most reliable way to check whether your baby has a fever. A rectal reading reflects your baby’s core body temperature, which is what pediatricians use to make decisions about infants under 3 months old. The process takes less than a minute once you’re set up, and with the right technique, it’s safe and straightforward.
Why Rectal Is the Most Accurate Method
Other methods, like armpit (axillary) readings, can differ from a rectal reading by up to 1°C (about 1.8°F). That might not sound like much, but a baby’s normal temperature range is narrow, between 97.7°F and 99.5°F (36.5°C to 37.5°C). A difference of even one degree can mean the difference between a normal reading and one that signals a fever. Studies show that armpit readings miss about 35% of elevated temperatures that a rectal thermometer would catch.
For babies under 3 months, accuracy matters most. A rectal temperature of 100.4°F (38.0°C) or higher in this age group is a specific threshold that requires immediate medical evaluation. An unreliable reading from another method could either cause unnecessary panic or, worse, miss a fever that needs urgent attention.
What You’ll Need
- A digital rectal thermometer. Use a standard digital thermometer with a flexible tip. Never use a glass or mercury thermometer. Designate one thermometer for rectal use only and label it so it doesn’t get mixed up with oral thermometers.
- Petroleum jelly. A small dab on the thermometer tip makes insertion comfortable and safe.
- Rubbing alcohol or soap. For cleaning the thermometer before and after use.
- A clean diaper and wipes. Rectal stimulation sometimes triggers a bowel movement, so have a fresh diaper ready.
Step-by-Step Instructions
Clean the thermometer tip with rubbing alcohol or soap and cool water before you start. Turn it on and make sure it’s reading and ready.
Apply a small amount of petroleum jelly to the silver tip of the thermometer. This acts as a lubricant and helps the thermometer slide in without friction or discomfort.
Position your baby. The two safest positions are:
- Belly-down on your lap. Lay your baby face-down across your thighs, with one hand placed firmly on the lower back to keep them still. This is the most common position and gives you a clear view.
- On their back. Place your baby face-up on a firm surface and gently lift both legs toward the chest, the way you would during a diaper change. Hold the legs steady with one hand.
Gently insert the lubricated thermometer tip into the rectum about half an inch to one inch (1.3 to 2.5 cm). Do not push past one inch. The anatomy of an infant’s rectum curves forward, and inserting deeper than 2 cm can press the thermometer against the front rectal wall. If you feel any resistance at all, stop.
Hold the thermometer in place between two fingers, with your palm resting against your baby’s bottom. This lets you keep the thermometer steady while also feeling any movement. Never let go of the thermometer while it’s inserted.
Wait for the beep. Most digital thermometers take 10 to 20 seconds to register a final reading. Keep your baby calm and still during this time. Gently remove the thermometer when you hear the signal.
Reading the Results
A normal rectal temperature for a baby falls between 97.7°F and 99.5°F (36.5°C to 37.5°C). A rectal reading of 100.4°F (38.0°C) or higher is considered a fever.
Context matters by age. For babies under 3 months, any fever of 100.4°F or above needs prompt medical attention, even if your baby seems fine otherwise. For children of any age, a temperature that repeatedly climbs above 104°F (40°C) is a concern. A fever lasting more than 24 hours in a child under 2, or more than 72 hours in a child 2 and older, also warrants a call to your pediatrician.
Cleaning the Thermometer
After each use, wash the thermometer probe with soap and cool water, then wipe it down with rubbing alcohol. Rinse again with cool water and let it air dry. Don’t use hot water, which can damage the digital sensor. Store it in its case, clearly labeled for rectal use only.
Common Concerns About Safety
Rectal perforation from a thermometer is extremely rare, and the cases documented in medical literature are almost exclusively linked to old glass thermometers, excessive insertion depth, or forceful technique. With a modern flexible-tip digital thermometer, gentle technique, and an insertion depth of no more than one inch, the risk is essentially zero.
The key safety rules are simple: never force the thermometer, never insert it more than one inch, and never let go of it while it’s in place. If your baby is squirming too much for you to hold the thermometer steady, remove it and try again when they’re calmer. A squirming baby with an unsecured thermometer is the one scenario to genuinely avoid.
When to Switch to Other Methods
Rectal temperatures are most important for babies from birth through about 3 months, when fever screening needs to be precise. Once your child is around 3 to 6 months old, you can begin using other methods like temporal artery (forehead) or axillary thermometers for routine checks. By age 4, most children can cooperate with an oral thermometer. If you ever get a borderline or surprising reading from a non-rectal method in a young child, though, a rectal check remains the best way to confirm.