BBT tracking for fertility

June 6, 2022
by
Lindsay Meisel
Head of Content at Ava Women
in
Fertility care

This guest blog post is from Carrot’s partner Ava Women. The Ava bracelet identifies five out of the six most fertile days of the cycle. Carrot members whose plan includes Ava can learn more about accessing Ava in their Carrot account.

Tracking your basal body temperature can be a helpful method for understanding your fertility at home. Many people gravitate toward this method because it is inexpensive, easy to learn, and can help pinpoint the timing of ovulation.

In this article, we provide a complete overview of using basal body temperature to track fertility, including the pros and cons of this method, the research behind its efficacy, and how it compares to other methods.

What is basal body temperature?

Your body temperature follows a circadian rhythm, meaning it varies over the course of the day. The lowest point it reaches over a 24-hour period is called the basal body temperature (BBT).

In people who ovulate, BBT changes throughout the menstrual cycle. During the follicular phase — the phase before ovulation — BBT is lower. After ovulation, progesterone levels increase, which causes BBT to increase by 0.5 – 1.0 degrees Fahrenheit. BBT then remains elevated for the duration of the luteal phase, which follows ovulation.

What is BBT tracking?

BBT rises in response to the increase in progesterone after ovulation. Charting your BBT, either on paper or with a fertility tracking app, can help you visualize this pattern and estimate when in your cycle you ovulate.

Knowing when you ovulate isn’t the same as knowing when you’re fertile. The fertile window of the menstrual cycle is the five days before ovulation, the day of ovulation, and up to 12 – 24 hours after ovulation. BBT tracking can help you determine when you ovulated after the fact.

If you have regular menstrual cycles, you can use this information to estimate when you might be fertile the following month. It can also be helpful in determining when to expect your period to start and when you can accurately take a pregnancy test.

How to track basal body temperature

To estimate your BBT, take your temperature first thing every morning, before you talk, drink water, or get out of bed. It’s best to use a digital oral thermometer or one that is specially designed for BBT tracking.

For the most accurate results, take your temperature at the same time each morning before getting out of bed and after at least three hours of uninterrupted sleep. Record your daily temperatures on a paper chart or with a fertility tracking app and look for a biphasic pattern to emerge. Remember that the overall pattern is more important than any one night of data. Ovulation has likely occurred when your BBT remains elevated for at least three days in a row.

Is BBT tracking accurate?

Despite its popularity, the research indicates that oral BBT tracking is not the most accurate method for determining ovulation. Even for fertility tracking experts, there is not always agreement on how to interpret a BBT chart and identify the first day of rising temperatures. One study found that in 78% of cycles, the day of ovulation determined by BBT did not coincide with timing based on tracking the LH surge before ovulation. A 2005 review article concluded that BBT tracking should not be recommended to people planning a pregnancy.

Benefits and disadvantages of BBT tracking

BBT tracking is a low-cost, accessible method people can use at home to track fertility. It can help you understand your body and know when to expect your period. The main downside of tracking BBT is that on its own, it’s not very useful for tracking fertility. That’s because temperature only rises after ovulation has occurred, when the fertile window is essentially already over.

Temperature is best used in combination with other fertility signs, such as LH tests and cervical mucus observations.

Benefits

Disadvantages

Low cost, at-home method

Requires consistency 

Know when to expect your period

Accuracy is impacted by lifestyle factors such as illness, alcohol consumption, poor sleep

Know when to take a pregnancy test

Low accuracy for determining ovulation

With regular cycles, can be used to estimate fertile window of subsequent cycles

Does not detect the fertile window of the current menstrual cycle

Resources for Carrot members

If you’re a Carrot member and have questions about BBT tracking, you can schedule a chat with one of our fertility experts to learn more. Depending on where you live, you may also have access to a wearable fertility tracker called the Ava bracelet, which combines temperature with other physiological signals to detect the fertile window. The Ava bracelet measures wrist skin temperature, which one study found to be more sensitive than oral BBT tracking for detecting ovulation.

While BBT is measured at one moment in time, the Ava bracelet records data continuously throughout the night. This continuous measurement makes it less susceptible to variations in waking time compared to oral BBT tracking.

Carrot members whose plan includes Ava can learn more about accessing Ava in your Carrot account.

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