Rectal thermometers are the most reliable method for taking a baby's temperature. The rectal thermometer gives consistent, accurate results and can be used on children who are too young to hold an oral thermometer. Each part of your baby's body yields a different normal temperature. When measuring temperature with a rectal thermometer, anything exceeding 100.4 degrees Fahrenheit is considered a fever, according to Tylenol.com.
Selection
Baby Center recommends choosing a rectal thermometer with a flexible tip and a wide handle. This keeps you from accidentally inserting the thermometer too far. Once you use a thermometer rectally, wrap a piece of tape around it or make a small mark on it with permanent marker so you can always use the same thermometer for taking rectal temperatures.
Prep
Clean the end of the thermometer with warm, soapy water, then rinse it with cool water. Coat the end of the thermometer lightly with petroleum jelly, or slide it into a disposable sanitary sleeve designed for rectal insertion.
Position Baby
Position your baby stomach-down across your lap or on his back on a flat surface. Place a towel or sheet beneath him--inserting the thermometer stimulates his bowels, and it's not unusual for him to defecate afterward.
Insert Thermometer
Spread your baby's buttocks apart gently with one hand if he's on his belly. If he's lying on his back, grasp both legs in one hand and lift them slightly as if you were changing his diaper. Press the button to turn the digital thermometer on and slide the thermometer tip no more than 1 inch into his rectum. The tip of the thermometer should be completely hidden from view. Hold onto both the baby and the thermometer.
Read Temperature
Wait until the thermometer beeps, which takes between 30 seconds and two minutes, depending on the thermometer. Remove the thermometer and read your baby's temperature. Write it down if you're not sure you can remember it. Discard the disposable sleeve and set the thermometer aside on a clean cloth, if necessary, until you've dealt with any unexpected bowel movements. Clean the end of the thermometer with soap and warm water and rinse before putting it away.
