july 2022

How To Track Basal Body Temperature (BBT)

10 min read Malena Monteverde Recommended Products
Body temperature

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It can be hard to consistently track where you are in your menstrual cycle and when you are ovulating. Your basal body temperature (BBT) may help you understand your fertility for natural family planning. Tracking your BBT is a popular method, and it is relatively quick and easy to do with just a thermometer. This article will provide you with all the information you need about tracking your basal body temperature to support you when you are trying to conceive.

What is basal body temperature?

Your basal body temperature (BBT) is your body's temperature when it is entirely at rest. It rises by 0.2-0.6°C when you ovulate, and this is caused by the rise in your hormone progesterone. Your BBT will remain higher for the rest of your cycle and drop just before your period starts, unless you are pregnant. If you have become pregnant, your progesterone levels will remain high and, thus, so will your BBT. Because BBT increases once ovulation has occurred, you will typically be most fertile for two-three days before your temperature rises.

Tracking and measuring BBT can provide you with vital information about where you are in your menstrual cycle, giving you a good indication of when ovulation is taking place every month and, also, if you have conceived. It is essential to note, however, that you need to track your BBT for approximately 3 months so you get a pattern of temperature changes throughout your cycle. This can then help you predict when you are approaching ovulation and, thus, when you are in your fertile window (the days preceding ovulation).

person holding thermometer

Choosing a BBT chart

The first step to tracking your BBT is to choose a basal body temperature chart. A chart is a record of the temperature measurements you’ve made every day over your menstrual cycle. Charting your BBT will allow you to see changes and patterns throughout your cycle and help you identify when you are in your most fertile days.

Our midwife-developed Ovulation Chart is downloadable and printable, and it is also included in our not-for-profit Trying to Conceive Kit. BBT charts can also be found in some fertility books, online and as part of fertility apps. Alternatively, you could create your own chart by using squared paper and plotting the temperature range along the vertical axis - a tenth of a degree for each square - and the days of the cycle on the horizontal axis. For ease and to avoid mistakes, though, most women prefer to use an already existing chart.

How can I track my BBT?
Tracking your BBT is a simple process yet, to ensure accuracy, you will need to be consistent and follow certain rules. You will need to

  • Take and note your temperature every morning before getting out of bed. You can track your temperature using a regular thermometer or a special basal thermometer used to measure BBT. Ensure you always take your temperature in the same way (for example, orally) for an accurate reading.
  • Get at least three hours of uninterrupted sleep. The results are likely to be more accurate.
  • Take the temperature at the same time every day, or as close as possible to the same time - ideally within 30 minutes. Setting an alarm clock or a reminder can be useful.
  • Record your temperature on the chart. Over time you will see a pattern. If your recorded temperature has changed by an average of 0.4 degrees Celcius in 48 hours over three days, ovulation has likely occurred.