After a baby is born, hospital nurses take armpit temperatures. They do this for two reasons. First, they want to be sure a baby’s temperature doesn’t drop as she adjusts to being outside the uterus. Second, because the nurses take temperatures multiple times per day, they do it in a way that is quicker and less disruptive for the baby.

Once you take your baby home, you should take rectal temperatures if you’re concerned that the baby is sick or has a fever. The reason for this is because rectal temperatures are the most accurate way to check for fever, and a doctor’s diagnostic approach to fever is based on rectal temperatures. Pediatricians define fever in the first three months as a rectal temperature of 100.4 degrees or higher.

It’s much easier to take rectal temperatures on newborns than older infants. There are different ways to do this, but I prefer having a baby lying across my legs with her bottom facing up. I separate her buttocks with one hand and gently insert a lubricated digital thermometer with the other. I insert the tip about ½ inch and hold the thermometer in place with two fingers of the same hand I used to separate the baby’s buttocks. That way I don’t have to worry that the device will poke the baby if she wiggles or moves.