In this tutorial, we learn how to use a rectal thermometer with a baby. This is the best to use for an infant six months or younger. Any fever could be serious, especially in the first 3 months. First, apply petroleum jelly to the baby's rectum and lift its legs in the air. Gently slide the thermometer into the rectum and wait until the temperature reads out. This will often stimulate the bowel movements of the baby, so make sure you keep wipes or paper towels near by for when you're finished taking the temperature. If there is a slight change in fever, see the doctor to be safe.