What is basal body temperature, and what can it tell you?
Basal body temperature (BBT) is your body's resting temperature, measured right after waking before you move. After ovulation, progesterone raises it by about 0.2 to 0.5 °C (0.4 to 1 °F), and it stays higher until your period. A sustained rise on a BBT chart suggests ovulation has already happened. It confirms the past rather than predicting ovulation ahead.
How BBT changes across your cycle
| Cycle phase | Typical BBT |
|---|---|
| Period and follicular phase | Lower: roughly 35.5 to 36.6 °C (96 to 98 °F) |
| After ovulation (luteal phase) | Higher by about 0.2 to 0.5 °C (0.4 to 1 °F), roughly 36.1 to 37.2 °C (97 to 99 °F) |
| Just before your period | Usually drops back down |
| If pregnant | Often stays high past the day your period was due |
The rise comes from progesterone, which the ovary releases after an egg is set free. Progesterone also drives many luteal-phase feelings, from sleepiness to PMS, so a BBT chart and a mood log often line up neatly. See temperature before period.
How to take your basal body temperature
- Use a BBT thermometer that reads to two decimal places.
- Take it at the same time every morning, before you sit up, talk, drink or check your phone.
- Aim for at least 3 hours of unbroken sleep first.
- Use the same method each time (mouth is most common).
- Log it daily and look for a rise of at least 0.2 °C that holds for 3 days in a row.
- Note anything that could skew it: alcohol, illness, travel, a late night.
Smartwatches like Apple Watch measure skin temperature overnight, not BBT, and show changes from your own baseline. Apple describes its ovulation estimate as retrospective. See Apple Watch wrist temperature.
What BBT can and can't tell you
- Can: suggest ovulation probably happened, after the fact.
- Can: show your luteal phase length (rise to period), which is useful if you're trying to conceive. See short luteal phase.
- Can: hint at early pregnancy if it stays high for 18+ days.
- Can't: predict this cycle's ovulation in advance; by the time it rises, the most fertile days have passed.
- Can't: work well with shift work, broken sleep, illness or alcohol.
- Can't: prevent pregnancy on its own. Lulu is not contraception.
Over several cycles, BBT shows your usual ovulation timing, which helps with planning. Pair it with LH tests and cervical mucus for a fuller picture: see how to track ovulation.
See a doctor if your charts never show a temperature rise over several cycles, if your luteal phase is regularly under 10 days, if your cycles are very irregular, or if you've been trying to get pregnant for 12 months (6 months if 35 or older).
Frequently asked questions
What is a normal basal body temperature?
Before ovulation, roughly 35.5 to 36.6 °C (96 to 98 °F). After ovulation it's typically about 0.2 to 0.5 °C higher.
Does BBT rise before or after ovulation?
After. It usually rises a day or two after ovulation, once progesterone increases.
Can BBT tell if I'm pregnant?
A high temperature lasting 18 days or more after the rise can be a hint, but only a pregnancy test can confirm it.
Can I take BBT without a special thermometer?
A standard thermometer often isn't precise enough. A BBT thermometer reading to two decimals works best.
Sources
- Cleveland Clinic: Basal body temperature
- ACOG: Fertility Awareness-Based Methods of Family Planning
- Cleveland Clinic: Ovulation
This page is for information only and isn’t medical advice. If something worries you, please talk to a doctor.
Keep reading
How do you track ovulation?How to track ovulation: calendar estimates, LH tests, cervical mucus, temperature and wearables. What each tells you and how to combine them.How does Apple Watch wrist temperature work for cycles?Apple Watch wrist temperature tracks nightly changes from your baseline. How it relates to your cycle, how to read the chart, and what affects it.Does your temperature rise before your period?Your temperature before your period is usually a little higher, thanks to progesterone, then drops as bleeding starts. What's normal and what it tells you.
Ovulation calculator: when do I ovulate?Free ovulation calculator: enter your last period and cycle length to see your likely ovulation day for the next 4 cycles. Private, instant, estimates only.Apple Watch period trackers: what works with your watchWhich Apple Watch period tracker fits you? Apple Cycle Tracking, Natural Cycles, Clue, Flo, Stardust and Lulu compared on wrist temperature and watch use.