While there are many acceptable ways to take your child's temperature, you should use a rectal thermometer if you think that your baby has a fever. To take a rectal temperature, first wash and rinse the thermometer and shake it until it is reading below 96 degrees F. Next, rub a small amount of lubricant on it's tip and while your baby is lying down, hold one of your hands firmly on his lower back, just above his buttocks. Now gently insert the thermometer into his anus about 1/2 to 1 inches. Wait for two minutes and then remove the thermometer and slowly roll it until you are able to see the end of the mercury column, which is your baby's temperature. For infants under three months of age, a fever is considered to be a temperature over 100.4 degrees F (call your doctor immediately!).