Spread apart your baby's buttocks with one hand. Keep a firm grip on his buttocks by cupping them with the palm and fingers of the hand that's holding the thermometer. A digital thermometer measures temperature within 30 seconds to two minutes and then beeps when it's ready. If you're using a glass thermometer, try to keep the bulb in place for three full minutes. Beware that inserting anything into your baby's rectum can stimulate his bowels, so don't be surprised if he poops when you take the thermometer out.
Pediatricians consider rectal readings the gold standard of temperature-taking.
If you call your healthcare provider for advice about your baby's fever, you will probably be told to use the rectal method before your pediatrician will weigh in. Take an armpit reading by sitting close enough to your baby to put your arm securely around his shoulder. You can sit him on your lap or on a bed, or you can keep him relaxed and occupied by breastfeeding him. Make sure his underarm area is dry, and then slip the bulb under his armpit. To get a reading, the bulb needs to be in full contact with the skin, so hold his arm firmly against his side or bent and folded across his chest.