Place a thick slab of petroleum jelly on the base of the thermometer. • Lay baby on his stomach or hold his legs up near his ears while he's on his back. • Stabilize him so he doesn't wiggle too much -- with a hand on his back (if he's on his stomach) or holding his feet firmly (if he's on his back). • Insert the thermometer slowly into his anus about 1/2 inch -- just so that the metal bulb is inside. • Gently hold the thermometer steady between your index and forefinger. • Wait until it beeps and then pull the thermometer out. Any temperature of 100.4 Fahrenheit and higher in a young baby is considered a fever and a reason to call your doctor immediately. take an oral temperature in your baby • Turn on the digital thermometer. • Place the tip of the thermometer under your child's tongue with his lips closed. • Remove the thermometer when it signals or beeps that it's done and read the number. If your child has been eating or drinking, wait at least 30 minutes to take his or her temperature by mouth. If your child is too congested to breathe through his or her nose, you might need to use another method.