How to Track Basal Body Temperature: Everything You Need to Know

Tracking basal body temperature is a popular and accessible method for pinpointing the timing of ovulation. In this post, we’ll explain how to track basal body temperature (BBT), share the pros and cons, the research on how accurate it is, and compare it to other methods of fertility tracking.

What is basal body temperature?

Body temperature follows a circadian rhythm over the course of a day. It’s highest in the early evening and lowest in the early morning around 4am. The lowest point your body reaches over a 24 hour period is called basal body temperature (BBT).

In women with ovulatory menstrual cycles, the circadian temperature pattern is layered atop a menstrual temperature pattern. BBT is lower during the pre-ovulatory follicular phase, and higher during the post-ovulatory luteal phase. In other words, BBT exhibits a biphasic pattern around ovulation. 

The reason for this change is that after ovulation, progesterone is released into the bloodstream, which causes BBT to increase slightly (0.5 – 1.0 degrees Fahrenheit). BBT remains elevated until progesterone levels fall and the next menstrual cycle begins (this can happen just before menstruation or several days into bleeding). In the case of pregnancy, progesterone levels and BBT remain elevated. 

Why to track basal body temperature

Charting your BBT, whether on paper or with a fertility tracking app, can help you estimate when ovulation occurs in your menstrual cycle.

Knowing when you ovulate isn’t the same thing as knowing when you’re fertile. The fertile window is the five days before ovulation and the day of ovulation. Your BBT chart typically doesn’t indicate ovulation until several days after the fact, when your fertile window is already closed for the month.

Even though BBT can’t alert you to the opening of your fertile window, it’s still useful for the following reasons:

• If you have regular menstrual cycles, you can use your BBT chart to estimate when you might be fertile each month
• If you know when you likely ovulated, you can use this information to predict when your next period will start
• If you know when you likely ovulated, you can use this information to decide when to take a pregnancy test

Some people claim that specific BBT patterns can reveal health conditions like hypothyroidism or low progesterone, but research in these areas is lacking.

How to track basal body temperature

There are three basic steps to tracking your BBT:

1. Take your temperature first thing in the morning, before you talk, drink water, or rise from bed.
2. Record your temperature on a BBT chart, either on paper or in an app.
3. Analyze your results. You can do this yourself or let an app do it for you.

For the most accurate results, follow these guidelines:

• Use a digital oral thermometer or one that is specially designed for BBT tracking. You can also use a wearable fertility tracking device like the Ava bracelet.
• Try to take your BBT at the same time each morning.
• Try to get at least three hours of uninterrupted sleep before measuring your BBT

Record your BBT each day in a paper chart or 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.

You can take your temperature orally, vaginally, or with a wearable device. Most women track BBT orally, but some women find that vaginal temperatures show a clearer pattern around ovulation compared to oral temperatures.

Wearable devices provide an easier way to measure BBT, since you don’t have to remember to take your temperature manually each morning. And because they measure temperature continuously throughout the night, they may be more accurate than using a standard thermometer—see the next section for a fuller discussion.

If you decide to switch to a different way of measuring BBT, make sure to do it at the beginning of a new cycle as opposed to in the middle of a cycle, since vaginal temperatures are typically higher than oral temperatures, and wrist-worn temperatures are typically lower than oral temperatures. You want to be sure that you can see a temperature change related to ovulation, as opposed to a change in your measurement method.

Are wearable fertility trackers accurate?

There are now several wearable devices available to track your temperature. These devices remove the hassle of having to remember to take your temperature at the same time each morning. And a recent study found that one such device, the Ava bracelet, was more sensitive than oral BBT for detecting ovulation.

While BBT is measured at one point in time, the Ava bracelet records wrist skin temperature every 10 seconds during the night. The continuous measurement makes the Ava bracelet less susceptible to variations in waking time compared to traditional BBT.

How accurate is the BBT method?

Despite its popularity as a fertility tracking method, research indicates that oral BBT is not the most accurate method for determining ovulation.

When Natural Family Planning experts look at a BBT chart, they don’t often agree on how to define the first day of rising temperatures. One study found that in 78 percent of cycles, the day of ovulation determined by BBT did not coincide with the timing of the LH surge.

A 2005 review article concluded that BBT tracking should not be recommended to people planning a pregnancy. 

Bottom line? While BBT is not often able to pinpoint the precise day of ovulation on its own, it can help provide a more complete picture of your cycle when interpreted alongside other fertility signs like cervical mucus and LH tests.

Benefits and disadvantages of BBT tracking

Tracking your BBT is a low-cost, accessible method for better understanding your menstrual cycle. The main downside with BBT tracking is that temperature only rises after ovulation, when the fertile window for that cycle is already closed.

Interpreting your BBT chart

The “three-over-six” rule is a common method for identifying ovulation on your BBT chart. With this method, when you observe three consecutive temperatures that are higher than the previous six temperatures, ovulation has occurred on the first day of higher temperatures. (Some practitioners still count this as ovulation if only five of the six preceding days are lower in temperature.)

It’s important to note that the three-over-six rule is not the only way to interpret a BBT chart, and some women have atypical temperature patterns including a slow BBT rise, or a pattern of rising and falling over several days before a conclusive rise.

This can make it more difficult to use BBT to determine the precise day of ovulation, but it’s still usually possible to tell if a cycle is ovulatory by looking at the overall pattern.

Lindsay Meisel

Lindsay Meisel is the Head of Content at Ava. She has over a decade of experience writing about science, technology, and health, with a focus on women's health and the menstrual cycle. Her work has been featured on The Fertility Hour, The Birth Hour, The Breakthrough Journal, and The Rumpus.