A rectal thermometer is the most accurate way to take a temperature. To take a rectal temperature: Apply a lubricant jelly or petroleum jelly, such as Vaseline, on the bulb of the thermometer, so that you can insert it easily.

What do I put in my baby's rectal thermometer?

Put some petroleum jelly on the end of the thermometer and the anus. Slide the thermometer gently into the anus no more than 1 inch. If your child is less than 6 months old, put it in no more than ½ inch. That means until you can no longer see the silver tip.

Does rectal thermometer hurt baby?

Parents worry about these because they think they may insert them incorrectly. But there is little risk for injury if you use a thermometer made for taking a rectal temperature.