Body temperature follows a circadian rhythm, meaning it varies over the course of the day. Your basal body temperature (BBT) is the temperature of your body when you’re completely at rest or your lowest body temperature.
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 is the phase after ovulation.
Tracking BBT can be a helpful method for understanding fertility at home. Many gravitate toward this method of family planning because it’s inexpensive, easy to learn, and can help pinpoint the timing of ovulation.
What is BBT tracking for fertility?
BBT rises in response to the increase in progesterone after ovulation. Charting BBT, either on paper or with a fertility tracking app, can help visualize this pattern and estimate when in a cycle you ovulate.
Knowing when ovulation occurs 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 helps you determine when you ovulated after it happens.
If you have regular and consistent menstrual cycles (occurring approximately every 21–35 days), you can use this information to estimate when you might be fertile the following month. It can also help determine when to expect your menstrual cycle to start.
How to track BBT
1. Take your temperature at the same time every morning
Take your temperature at the same time each morning before getting out of bed and after at least three hours of uninterrupted sleep. While a regular digital thermometer can work, it’s best to use a basal thermometer. Unlike a regular thermometer, a basal thermometer can measure small changes in temperature.
2. Chart your daily temperatures
Record your daily temperatures on a paper chart or with a fertility tracking app to see how your temperature fluctuates throughout the month. It’s best to start taking BBT on the first day of your menstrual cycle and continue to take it every morning until the start of your next menstrual cycle. This will help establish a baseline temperature before ovulation, so you can identify the temperature shift at ovulation.
Ovulation has likely occurred when your BBT remains elevated for at least three days in a row. This is due to the increase in progesterone.
3. Identify ovulation
Track your BBT for at least three menstrual cycles. Once you have identified your ovulation pattern, you can predict your most fertile days for future cycles. If you’re hoping to get pregnant, you are most fertile two days before BBT rises, which would be the most optimal time to have sex.
Benefits and disadvantages of BBT tracking
BBT tracking is a low-cost, accessible way to become more aware of your fertility and know when to expect your menstrual cycle. If you have regular menstrual cycles and are consistent and diligent in tracking BBT, this method can be a useful tool to pinpoint ovulation better. However, it does have some limitations. A rise in BBT confirms that ovulation has occurred after the fact. Therefore, it may not be the most effective method for determining the exact day of ovulation.
In fact, fertility tracking experts do not always agree 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 luteinizing hormone (LH) surge before ovulation.
However, BBT tracking combined with other methods such as LH test strips or monitoring changes in cervical mucus can increase the effectiveness of BBT in determining ovulation. If you’re considering BBT tracking for family planning purposes, consult a healthcare provider to ensure you use the method correctly and effectively.
BBT fertility tracking devices
BBT fertility tracking devices, such as watches, rings, and bracelets, are popular for more precise ovulation tracking. Many products, such as smart watches and rings, now have BBT fertility tracking built into their existing health trackers, making this method of fertility awareness accessible, convenient, and easy to use.
Unlike recording BBT daily with a thermometer, a wearable device automatically collects temperature while you sleep. This collection of continuous data can reduce possible user error from manually taking BBT and can provide helpful insight into fertility. However, these types of fertility tracking devices can be costly.
Can you use a normal thermometer for BBT?
You can use a regular digital thermometer, but may not get the most accurate results. The increases in temperature during ovulation are very slight, typically about 0.5–1.0 degrees Fahrenheit. Unlike a regular digital thermometer, a basal thermometer displays temperature in tenths of a degree, allowing you to note subtle changes in body temperature. This will provide a more exact reading than a regular digital thermometer.
Are there factors that can affect my BBT?
Many factors can affect BBT, including:
- A fever or other illness
- Interrupted sleep cycles or oversleeping
- Drinking alcohol
- Jet lag
- Breast/chest feeding
- Taking some medications
Can BBT tracking be used as a form of birth control?
BBT tracking is not the most effective method of birth control, especially if your cycles are irregular. In addition to ovulation, other factors can affect BBT, including jet lag, stress, and the common cold. This can make tracking BBT for pregnancy prevention unreliable. If you don’t want to get pregnant, it’s recommended to use other forms of birth control in addition to BBT tracking.
Can BBT tracking help in detecting pregnancy?
BBT can be an indicator of early pregnancy. After ovulation, an increase in BBT that lasts more than 18 days may be a sign of pregnancy.
Other ovulation tracking methods
BBT tracking is a low-cost and accessible method to become more aware of your fertility and ovulation. However, BBT tracking is just one method of tracking ovulation. There are other ovulation tracking methods that may be better suited for different lifestyles and needs.
Ovulation predictor kits (OPKs) are another popular way to monitor ovulation. OPKs are urine tests that can be done at home to detect ovulation. These test strips detect the levels of LH in the urine. Because LH surges about 36 hours before ovulation, OPKs are helpful in predicting the fertile window.
Resources for Carrot members
If you’re a Carrot member and have questions about BBT tracking, talk to one of our fertility experts to learn more. Your Carrot Expert can also share additional ovulation tracking resources you may have access to through Carrot.