To ensure an accurate reading, use a digital rectal thermometer on infants, recommends Dr. Ashanti Woods, a pediatrician at Mercy Medical Center in Baltimore. "These thermometers are accurate and display temperature in under 20 seconds," he says. Avoid temporal artery, forehead or ear canal thermometers, as they are often inaccurate due to user error, he states. Before and after each use, clean the tip of the thermometer with rubbing alcohol or soap and water. Lubricate the tip of the thermometer with petroleum jelly and, with baby lying safely on his back, insert the tip about 1/2 inch to 1 inch into his rectum and hold it still until it beeps. Call the doctor if the thermometer reading is higher than 100.4 degrees F for a baby younger than 2 months of age, or higher than 102 for a baby older than 2 months.