A rectal thermometer is the most reliable method for checking an infant’s core body temperature, and a reading at or above 100.4°F (38°C) signals a fever in babies.
Knowing how to take a rectal temperature in your baby gives you the most accurate core body reading available for infants and young children. While armpit and ear thermometers are convenient, only a rectal measurement reflects the true internal temperature — which is why pediatricians and hospitals rely on this method for the clearest fever picture.
Why Rectal Temperature Is Recommended for Babies
An infant’s small body surface and immature temperature regulation make less invasive methods unreliable. Oral readings are impractical for babies who cannot hold a thermometer under their tongue, and ear or armpit measurements can miss a fever by a full degree. A rectal thermometer bypasses these issues by reading temperature directly from the body’s core. The typical normal range for a rectal reading in babies is 97.6°F (36.4°C) to 100.2°F (37.9°C). Per the guidelines on KidsHealth, any reading of 100.4°F or higher constitutes a fever and warrants attention, especially in infants under 12 weeks old.
Step-by-Step: How to Take a Rectal Temperature
The procedure takes less than two minutes once you have the supplies ready. Use only a digital thermometer designed for rectal use — never glass or mercury models. A small amount of petroleum jelly makes insertion safe and comfortable.
| Step | What to Do |
|---|---|
| 1 | Wait 20–30 minutes after a bath, heavy bundling, swaddling, or multiple layers before measuring. |
| 2 | Wash the thermometer tip with soap and water or rubbing alcohol. |
| 3 | Apply a small amount of petroleum jelly or other lubricant to the tip. |
| 4 | Place your baby belly-down across your lap or on a firm, flat surface with your hand on the lower back — or face-up with legs bent toward the chest. |
| 5 | Gently insert the lubricated tip into the anus about 1/2 inch to 1 inch (1.25 to 2.5 cm), pointing toward the baby’s belly button. Stop if you feel any resistance. |
| 6 | Hold the thermometer steady until it beeps or signals completion, then read and record the temperature and time. |
| 7 | Clean the thermometer again with soap and water or rubbing alcohol, and label it for rectal use only. |
Having a dependable digital thermometer makes the whole process smoother — take a look at our guide to the best baby rectal thermometers for reliable options that parents trust.
Safety Points and Common Mistakes
Most problems with rectal temperature taking come from a few avoidable errors. Never force the thermometer past resistance — if you meet firm resistance, remove it and try again after a moment. Always keep one hand on your baby during the measurement, and never leave an infant alone with a thermometer inserted. The thermometer tip should point toward the child’s belly button, not straight in, which follows the natural angle of the rectal canal.
Common mistakes that produce inaccurate readings include skipping lubrication, measuring too soon after a bath or bundling, inserting the thermometer too far, and using the same thermometer for both mouth and rectum without labeling it. Glass or mercury thermometers should never be used for rectal measurements — digital thermometers are safer and faster. If your baby’s rectal temperature reads 100.4°F (38°C) or higher and your infant is under 12 weeks old, call your pediatrician promptly.
FAQs
What temperature is considered a fever in babies on a rectal thermometer?
A rectal reading of 100.4°F (38°C) or above is classified as a fever in infants. This is the threshold pediatricians use because rectal measurements most accurately reflect core body temperature, making it the standard for fever diagnosis in babies.
Can I use the same digital thermometer for rectal and oral readings?
You should not use the same thermometer interchangeably without careful labeling. Designate one thermometer for rectal use only and clearly mark it so it is not later used for oral readings. Using separate or distinctly labeled thermometers prevents the spread of bacteria and keeps each reading accurate.
How long should I wait to take a rectal temperature after my baby eats or bathes?
Wait at least 20 to 30 minutes after a bath, heavy bundling, swaddling, or feeding before taking a rectal measurement. These activities can temporarily raise or affect your baby’s body temperature, and measuring too soon can give a false reading that does not reflect the true core temperature.
References & Sources
- KidsHealth. “Taking a Rectal Temperature.” Detailed step-by-step instructions for parents on safe rectal temperature measurement.
- Mayo Clinic. “Thermometer basics: Taking your child’s temperature.” Authoritative guide on thermometer types, accuracy, and fever thresholds for children.