Positioning Gauge: Some rectal thermometers feature an indicator that lets you know it isn't inserted too far -- a definite plus for peace of mind.
Sound Effects: A thermometer that beeps when it's done is useful; that way, you won't leave it in any longer than necessary.
Brains: Look for a thermometer that "remembers" the previous temperature taken and displays it when you push a button. That function can be handy, especially in the middle of the night, when you're sleep-deprived and trying to assess in which direction your baby's fever is going. Temps You can take a baby's temperature many ways -- by mouth or ear, on the forehead, or under the arm -- but the American Academy of Pediatrics considers rectal readings to be the most precise in children younger than 3.
Your best bet: Get an inexpensive digital thermometer that can be used rectally. Use a disposable probe cover each time, or clean the end with rubbing alcohol and rinse with cool water after each use. Once your baby has her first birthday, your pediatrician may allow you to use a different temp-taking method; ask.