There are generally three ways to measure the temperature of a 40-day-old baby, respectively, using a thermometer in the anus, mouth, armpits and other locations. 1. Anal: 40-day-old baby parents can use the thermometer gently inserted into the baby’s anus, the general normal anal temperature of 36.5 ℃ to 37.5 ℃, if not in the above range of values is considered fever. 2. Mouth: insert the thermometer into the baby’s mouth, you can also measure the baby’s body temperature, the general normal oral temperature is 36.2 ℃ to 37.3 ℃. 3. armpit: armpit refers to the temperature of the baby’s armpit, generally normal body temperature in 35.9 ℃ to 37.2 ℃ or so. If a 40-day-old baby measures the above parts and is out of range, it is recommended to go to the hospital in time for further examination and active treatment.