Rectal temperature is the most accurate way to measure core body temperature, especially for infants under 3 months old. The process takes about a minute, requires a digital thermometer and petroleum jelly, and involves inserting the thermometer tip only 1/2 to 1 inch into the rectum.
Why Rectal Temperature Is Recommended
Rectal readings reflect core body temperature more closely than oral, forehead, or underarm methods. For babies and toddlers, other methods can be unreliable because a squirming child makes it hard to get a consistent reading. The American Academy of Pediatrics considers rectal measurement the best option for infants under 3 months and a reliable choice for children up to about age 3.
What You Need
Use a digital thermometer. Glass mercury thermometers are no longer recommended because mercury is toxic if the thermometer breaks, posing a serious health risk if inhaled or if the liquid touches skin. Many pharmacies sell digital thermometers labeled specifically for rectal use, which have a shorter, flexible tip. If you use a standard digital thermometer, designate it for rectal use only and label it so no one accidentally uses it orally.
You’ll also need petroleum jelly (or another water-based lubricant) and rubbing alcohol or soap and water for cleaning afterward.
Step-by-Step Instructions
Turn on the digital thermometer first so it’s ready before you position your child. Apply a small dab of petroleum jelly to the silver tip.
You have two positioning options:
- Back position: Lay your baby on their back on a flat, stable surface. Gently lift both thighs upward with one hand, the way you would for a diaper change. With your other hand, gently insert the lubricated tip into the rectum.
- Belly-down position: Place your child face-down across your lap or on a firm surface. Rest one hand on their lower back to keep them still, and use the other hand to insert the thermometer.
Insert the tip 1/2 to 1 inch (1.3 to 2.5 centimeters), no deeper. For newborns, stay closer to the 1/2-inch mark. Never force the thermometer past any resistance. If it doesn’t slide in easily, stop, apply more lubricant, and try again gently. If you still feel resistance, don’t continue.
Hold the thermometer in place between two fingers, with your hand cupped around your child’s bottom, so it can’t accidentally be pushed in further or pulled out by sudden movement. Wait for the beep indicating the reading is complete, which usually takes 10 to 20 seconds with most digital models. Gently remove the thermometer and read the display.
Taking a Rectal Temperature in Adults
The technique for adults follows the same principles. Lie on your side with knees drawn toward your chest. Apply petroleum jelly to the thermometer tip and insert it about 1 inch into the rectum. Hold it in place until the thermometer beeps. Adults rarely need rectal temperatures at home, but it’s sometimes used when someone is too ill for an oral reading or when maximum accuracy matters.
What the Reading Means
A normal rectal temperature runs about 0.5 to 1°F (0.3 to 0.6°C) higher than an oral reading. A rectal temperature of 100.4°F (38°C) or above is generally considered a fever. This is the threshold most pediatricians use for infants, and it’s the number to reference if you’re calling a doctor about a sick baby.
For infants under 3 months, a rectal temperature at or above 100.4°F warrants an immediate call to your pediatrician, even if the baby seems otherwise fine. Fever at that age can signal a serious infection that’s harder to detect by symptoms alone.
Cleaning the Thermometer
After each use, wash the thermometer tip thoroughly with soap and warm water. You can also wipe the tip with rubbing alcohol, which is effective at disinfecting thermometers. Let it air dry or wipe it clean before storing. Always clean the thermometer before and after each use, even if it looks clean.
When Not to Use a Rectal Thermometer
Rectal thermometry is safe for the vast majority of children and adults, but there are situations where it should be avoided. Children who are immunocompromised, have bleeding disorders, or have had recent rectal surgery or trauma should not have their temperature taken rectally. In these cases, an underarm or temporal artery (forehead) reading is a safer alternative, even though it’s slightly less precise. If your child has any condition affecting the rectal area, ask your pediatrician which method to use instead.