Your basal body temperature (sometimes shortened to BBT) is your resting temperature, measured first thing in the morning before you get up, move around, or drink anything. 

After you ovulate, progesterone raises your baseline temperature slightly, and that sustained rise is what a BBT chart is looking for. It's one of the most-recommended and most-misunderstood tools in fertility tracking. You might have seen it framed as an ovulation predictor, but it isn't – BBT confirms ovulation after it’s happened, not before.

A sustained temperature shift held over several days is strong evidence that ovulation has happened, and if you keep track of your basal body temperature over a few cycles, you can get a good idea of whether you're ovulating regularly, roughly when in your cycle it tends to happen, and how long your luteal phase (the second half of your cycle) usually runs. That's one of the clearest, most inexpensive ways to know your cycle is doing what it should. It’s also the kind of baseline information a fertility specialist would want to know, if you ever needed to have that conversation.

By the time your temperature rises, ovulation has already happened, and the fertile window (meaning when you could get pregnant) has closed for that cycle. If you're trying to conceive this month, BBT on its own isn't the right tool – for that, you want something predictive, like a luteinizing hormone tests (which detect the hormone surge that happens a day or two before ovulation) or cervical mucus tracking (which shifts as estrogen rises in the days leading up to it). Luteinizing hormone tests are available as “ovulation prediction kits” at your local pharmacy, if you’re curious. 

BBT works best paired with intent. If you're trying to conceive, pair it with ovulation prediction kits or mucus tracking to catch the fertile window in real time. 

If you just want to confirm that your body is ovulating and your luteal phase is a healthy length, BBT can carry that on its own. Either way, gather information across two or three full cycles before drawing conclusions – a single unusual chart doesn't mean much. The value of BBT is what it shows over time, not what it predicts at the moment.