BBT stands for basal body temperature, your body’s lowest resting temperature, typically measured first thing in the morning before you get out of bed. In pregnancy tracking, BBT is used to pinpoint ovulation and detect early signs of conception based on predictable temperature shifts driven by hormones. After ovulation, your temperature rises roughly 0.5 to 1 degree Fahrenheit and stays elevated. If you’re pregnant, it stays up. If you’re not, it drops back down right before your period.
Why Your Temperature Changes After Ovulation
The temperature shift is driven by progesterone. Right after you ovulate, the empty follicle in your ovary transforms into a temporary structure called the corpus luteum, which pumps out progesterone to prepare your body for a potential pregnancy. One of progesterone’s effects is acting on the temperature-regulating center of your brain to raise your baseline temperature. This is why BBT charting works: the rise confirms that ovulation has already happened.
Before ovulation, your resting temperature typically falls in a lower range. After ovulation, progesterone pushes it into a higher range, usually by about half a degree to a full degree Fahrenheit. This elevated phase is called the luteal phase and lasts an average of 14 days. If conception doesn’t occur, progesterone drops, your temperature falls, and your period starts.
How BBT Signals Early Pregnancy
The key signal is sustained high temperatures. In a non-pregnant cycle, your temperature stays elevated for roughly 10 to 14 days, then drops as the corpus luteum breaks down. In a pregnant cycle, the embryo sends a hormonal signal (hCG) that rescues the corpus luteum, keeping progesterone production going. Your temperature stays high instead of falling. If you’ve been charting and you see 18 or more consecutive days of elevated temperatures past ovulation without a period, pregnancy is very likely.
A luteal phase shorter than 12 days is considered short, and research shows it may be associated with slightly lower odds of conception in the following cycle. If you consistently see your temperature drop back down before day 11, that’s worth noting when you talk to a healthcare provider about fertility.
The Triphasic Pattern
Some pregnancy charts show what’s called a triphasic pattern: a third, distinct rise in temperature on top of the post-ovulation shift. This typically appears around 9 days past ovulation, which lines up with the most common window for implantation. The thinking is that the additional temperature bump may reflect the surge in hCG production or the rescue of the corpus luteum that happens when an embryo implants.
A large analysis by Fertility Friend found that about 12.5% of pregnancy charts showed a triphasic pattern, compared to only 4.5% of non-pregnant charts. So while a triphasic shift makes pregnancy more likely, it’s far from a guarantee. Most pregnant women never see one on their chart. It’s a positive sign if it appears, but its absence means nothing.
The Implantation Dip
You may also come across the idea of an “implantation dip,” a single-day temperature drop that occurs roughly 7 to 10 days after ovulation, before temperatures climb back up. The theory is that this brief dip corresponds to implantation. In practice, the evidence for it as a reliable pregnancy indicator is thin. Illness, poor sleep, stress, room temperature changes, and even getting up in the middle of the night can all cause a one-day temperature dip. A single low reading in an otherwise elevated chart is usually noise, not a signal.
How to Track BBT Accurately
BBT charting only works if you measure under consistent conditions, because the differences you’re looking for are small. You need a thermometer that reads to at least one-tenth of a degree (a regular fever thermometer often rounds too much). Dedicated BBT thermometers are inexpensive and widely available.
- Timing: Take your temperature at the same time every morning, ideally after at least three consecutive hours of sleep.
- Before moving: Measure before you sit up, talk, check your phone, or drink water. Keep the thermometer on your nightstand so you can reach it without getting up.
- Consistency: Use the same method each day, either oral or vaginal. Switching between them introduces variation.
- Record immediately: Log the reading right away, either on paper or in a charting app. Many apps will draw your temperature curve and flag ovulation and potential pregnancy patterns automatically.
Expect some daily noise. Individual readings bounce around because of alcohol, broken sleep, a stuffy room, or a mild cold. The pattern across days and weeks is what matters, not any single morning’s number.
What BBT Can and Can’t Tell You
BBT is useful for confirming that you ovulated and for spotting the sustained high temperatures that suggest early pregnancy. It’s a free, non-invasive way to understand your cycle, and many people use it alongside other fertility signs like cervical mucus changes.
What it can’t do is predict ovulation in advance. The temperature rise happens after the egg has already been released, so by the time you see the shift, your most fertile window has passed for that cycle. Over several months of charting, though, you can identify your personal pattern and anticipate when ovulation is likely to happen in future cycles. BBT also can’t replace a pregnancy test. A sustained temperature rise is an encouraging early clue, but a positive test is the only way to confirm pregnancy. Most home tests are reliable by the time your period is a day or two late, which roughly corresponds to 15 to 16 days of elevated temperatures on your chart.