The best thermometer for a newborn is a rectal thermometer because it provides the most accurate reading for babies 3 months or younger. To take their temperature rectally, you’ll need a little bit of lubricant (such as petroleum jelly) and a firm surface. Remember to disinfect the thermometer with soap and water or an alcohol wipe before using it. Put a small amount of the lubricant on the tip of your thermometer and turn it on. Your baby should be lying belly down across your lap or positioned on their back on a firm surface with their legs pulled up to their chest. Very carefully, insert the thermometer half an inch into their anus. You should not feel any resistance; if you do, stop. Keep your child still and the thermometer in place until the reading is finished. Then, you can remove the thermometer slowly. Be sure to disinfect the thermometer again when finished.