Where to Place Your Basal Body Thermometer
Place the thermometer tip in the posterior sublingual pocket -- lift your tongue and put it in the back, to the left or right of the frenulum (the membrane in the middle) -- not just anywhere under the tongue. Research found a 0.3-degree difference between placements.
You've hopped on the basal body temperature (BBT) train, ready to track your cycle with real precision instead of just eyeballing a calendar. But are you placing your thermometer in the right spot? Because where you place it matters more than you'd think.
The 0.3-degree difference
Researchers found a 0.3-degree difference in oral temperature readings depending on exactly where under the tongue the thermometer sits -- specifically, near the front just below the tongue versus further back.
The correct placement
Lift your tongue and place the thermometer tip in the back, to the left or right of the frenulum (the thin membrane running down the middle of your tongue's underside) -- this spot is called the posterior sublingual pocket. Close your mouth and wait for the beep. That's the whole trick.
You also need an accurate thermometer
If you've been taking your temperature but not getting consistent, readable results, it might not be your placement -- it might be your thermometer. A basic drugstore thermometer usually isn't precise enough to catch the subtle shift BBT charting depends on. A dedicated basal body thermometer, ideally one that's Bluetooth-enabled and syncs automatically to a charting app, removes a lot of the guesswork.
Reference
Erickson (1980). Nursing Research, 29(3), 157–164. Oral temperature differences in relation to thermometer and technique.
Common questions
Does thermometer placement really matter that much for BBT charting?
Yes -- a 0.3-degree swing is meaningful when you're looking for a subtle post-ovulation temperature shift, so consistent placement matters as much as consistent timing.
What kind of thermometer should I use for BBT?
A dedicated basal body thermometer with two decimal places of precision, ideally one that logs readings automatically so you're not relying on memory first thing in the morning.