The best way to get an accurate temperature reading on a child 5 years old or younger is rectally, using a digital thermometer. "A rectal temperature is the only true core body temperature, so parents should get comfortable using this method," says Dr. Leeds. You can position your child as if you were going to change his diaper (on his back with his legs raised) or lay him down on his belly, and then insert a digital thermometer a half inch into his anus and wait for the reading to be completed. If you're worried about hurting your baby, remember that the tip of the thermometer is much smaller than an infant's stool, says Dr. Leeds. Just don't use a mercury thermometer because if it breaks, the liquid mercury that spills out is toxic.
Taking an underarm temperature may be easier, but it's not particularly accurate, says Dr. Leeds, noting that these axillary readings tend to be 1.5 degrees lower than rectal readings.
Parents like ear thermometers because they're fast and easy to use, but they're also not as accurate as rectal readings.