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Illustration by Marta Pucci

Fertility

Basal Body Temperature (BBT): How is it used to estimate ovulation?

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Top things to know:

  • Your body's resting temperature rises slightly during the luteal phase (the second half) of your menstrual cycle

  • Carefully tracking your basal body temperature with a basal body thermometer will help you know when ovulation has occurred, but won't predict when it will happen in the future

  • Basal body temperature can be used in combination with cervical mucus changes to determine when fertility is highest

Basal body temperature (BBT) is your body's temperature at rest, like when you first wake up in the morning. Your reproductive hormones have a measurable impact on your temperature (1,2). A BBT thermometer is more sensitive than a regular household thermometer, and measures temperature more precisely.

Tracking your BBT is an easy way to get an idea of if and when you're ovulating. Using it as a part of fertility awareness requires additional symptom tracking, and not every Fertility Awareness Method (FAM) is right for every body — talk to your healthcare provider if you're interested in using a FAM for contraception.

Why track basal body temperature?

Tracking BBT with Clue can help make your predictions more accurate. BBT is used by Clue to assess when ovulation has occurred. However, BBT alone cannot predict when ovulation will occur in the future.

BBT is slightly lower in the follicular phase (the first half of the menstrual cycle), and rises after ovulation and stays raised throughout the luteal phase (the second half of the menstrual cycle) (2,3). This rise in temperature happens in response to progesterone, which is released after ovulation occurs. Progesterone prepares the uterus for the implantation of a fertilized egg. The temperature change after ovulation is slight — BBT rises by only about 0.5ºF/0.3ºC to 1.0°F/0.6ºC — and may be easily affected by factors such as illness, alcohol, and sleep changes (2,3).

Measuring BBT takes consistent effort over several cycles, including:

  • taking it immediately after waking, while laying still in bed, before sitting upright

  • taking it every day, at the same time

  • using a special thermometer with 0.10 degree accuracy for Fahrenheit (ºF) and 0.01 degree for Celsius (ºC), for precise rounding

  • making note of which days are "unreliable" due to environmental factors, such as changes in sleep patterns, disturbed or shortened sleep, jet lag, heavy drinking, illness, or medications, which can cause changes to your menstrual cycle (2,4,5)

BBT and Fertility Awareness Methods (FAM)

BBT is one component of certain Fertility Awareness-Based Methods (FAMs) of contraception, called symptothermal methods. By following rules that combine the changes in BBT with carefully-tracked changes in cervical mucus, the most fertile days of the cycle can be estimated (i.e. when someone has the highest likelihood of becoming pregnant) (6).

What's average and healthy

Temperature typically ranges from about 97°F/36.1°C to 99°F/37.2°C (7). Average basal body temperature will vary between individuals, and depending on the time of the day and the activity.

You should see your healthcare provider if…

  • You have a fever for longer than 48–72 hours, or have a fever that stays at or increases from 103˚F/39.4˚C

  • You suspect that you are pregnant

Download Clue and start tracking your basal body temperature.

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