Understanding Basal Body Temperature (BBT)
Your Basal Body Temperature (BBT) is your body’s lowest temperature at rest. When you
learn how to your Basal Body Temperature to track your ovulation you will learn valuable
insights into your menstrual cycle and your fertility.
Why Should You Track Your BBT?
Tracking BBT daily allows you to confirm ovulation during a menstrual cycle. Once ovulation
occurs, the follicle that released the egg collapses and forms the corpus luteum. This
temporary gland produces oestrogen and, more importantly, progesterone, which causes a
slight increase in your BBT. If ovulation doesn’t occur, progesterone is not produced, and
your temperature doesn’t rise. By monitoring these shifts, you can confirm ovulation with
100% accuracy.
What Temperature Shifts Indicate Ovulation?
Progesterone raises BBT by approximately 0.2-0.5°C (0.4-1.0°F). Though this may seem
minor, it results in an observable shift on a BBT chart. Women generally have two
temperature phases:
- Follicular Phase (Pre-ovulation): Lower temperatures of 36.1-36.4°C (97.15-97.5°F)
- Luteal Phase (Post-ovulation): Sustained higher temperatures of 36.5-37°C (97.6- 98.6°F)
Stability is key as you won’t get the exact same reading everyday. You should see steady,
lower temperatures in the follicular phase, followed by a sustained rise after ovulation. A
temperature increase that does not persist for at least three days does not confirm
ovulation. Remember: progesterone is only produced after ovulation and this hormone is
what causes the shift to higher temperatures.
How to Track BBT Accurately
Choosing the Right Thermometer
Use a basal body thermometer, which provides readings to two decimal places for accuracy.
Regular fever thermometers are not precise enough to detect the small temperature shifts
caused by progesterone.
Note: Basal body thermometers are not ideal for checking fevers. Keep a separate
thermometer for illnesses.
When to Start Tracking Your BBT
Begin tracking BBT on Day 1 of your cycle (the first full-flow day of your period). Continue
tracking daily until your next period.
If you don’t check your BBT daily, you may miss the shift in temperature and then miss
ovulation day.
Best Time to Take Your BBT
BBT must be measured first thing in the morning, before getting out of bed or engaging in
any activity. Consistency is essential:
- Take your temperature at the same time every day (even on weekends).
- Avoid movement, talking, eating, or drinking before taking your temperature.
- Keep your thermometer on your bedside table for easy access.
What If My BBT Is Not Stable?
Occasional fluctuations are normal, but it’s the overall trend that matters. Look for:
- Lower temperatures in the follicular phase
- A sustained temperature rise after ovulation (lasting at least three consecutive days to confirm ovulation)
Factors That Can Affect BBT Readings
Several factors can cause variations in your BBT, including:
- Sleep Disruptions: You need at least three consecutive hours of sleep prior to checking your temperature for an accurate reading.
- Alcohol Consumption: Drinking alcohol the night before can raise your BBT.
- Illness or Fever: Being sick can cause an artificial temperature spike.
- Stress: High-stress levels can influence body temperature.
- Travel & Shift Work: Changes in routine or sleep patterns may impact readings.
- Life Stages: Breastfeeding, perimenopause, and thyroid disorders can alter BBT patterns.
Using BBT for Conception
BBT confirms ovulation but does not predict it. Since the egg only survives 12-24 hours after ovulation, it is too late to conceive once BBT rises. However, sperm can live in the
reproductive tract for up to five days, so tracking other ovulation signs (like cervical mucus
and LH surges on ovulation tests) is essential for timing intercourse.
For more guidance, check out our other articles on ovulation tracking and fertility signs as
linked above.
The important point: If you are trying to conceive, using BBT tracking alone is not helpful.
Using the predictive ways of ovulation, you can better estimate your most fertile time in
your menstrual cycle to time sexual intercourse optimally for increasing your changes of
conceiving. Once you confirm ovulation, you are no longer fertile and can go back to having
sex for fun and intimacy in your relationship.
BBT Tracking for Irregular Cycles & PCOS
Women with long or irregular cycles can still benefit from BBT tracking, even though it can
be frustrating. Though ovulation may not occur regularly, charting BBT alongside cervical
mucus and ovulation tests can help identify fertile windows when the body is attempting
ovulation.
If you have long menstrual cycles, it is possible and even likely that you WILL ovulate, just
not regularly and predictably.
Tracking predictive signs of ovulation—such as cervical mucus changes and ovulation
tests—is essential for timing intercourse, as these indicators signal that your body is
preparing and attempting to ovulate. However, in women with irregular cycles, ovulation
doesn’t always occur on the first attempt, and your body may try multiple times before
successfully releasing an egg.
The advantage of tracking your basal body temperature (BBT) is that it allows you to confirm
when ovulation has actually taken place. If you’ve been timing intercourse based on your
predictive signs and have had sex in the five days leading up to ovulation, there’s a strong
chance that sperm will be present when the egg is finally released—giving you the best
opportunity to conceive.
How BBT Can Indicate Pregnancy
If a fertilised egg implants in the uterus, the pregnancy hormone HCG signals the corpus
luteum to continue producing progesterone. This results in sustained high BBT readings
beyond 16 days post-ovulation, indicating pregnancy. How cool is that, you will know
you’re pregnant even before you pee on the stick. Our body has all this information for us,
we just have to learn to understand it’s language.
If no pregnancy occurs, progesterone production will drop, and BBT falls 1-2 days before
menstruation.
Why BBT Tracking is a Powerful Tool
BBT tracking is an affordable and effective way to confirm ovulation and even detect
pregnancy. Though it requires diligence, the insights gained can empower you on your
fertility journey.
Many women feel a deep sense of relief and excitement when they can confirm that
ovulation has occurred. In the midst of what can be an emotional and unpredictable
journey, this certainty provides reassurance and a sense of progress.
Remember, BBT confirms ovulation, but it does not predict it is approaching. For
conception and trying to conceive, track other fertility signs like cervical mucus and LH
surges to optimise your timing of sexual intercourse.
If you’re ready to learn more about tracking ovulation and maximising your chances of
conception, explore our resources on fertility awareness and cycle charting!
Happy tracking!
Amanda xx