Wearable BBT: Is it just as accurate as oral BBT?

If you’ve ever struggled to remember to take your temperature when tracking ovulation, you may have wondered whether it’s possible to track your basal body temperature (BBT) with a wearable like the Ava bracelet. But how does wearable temperature compare to oral temperature?

It turns out that Ava’s wrist skin temperature (Ava-WST) comes out on top. That’s because the Ava bracelet is more sensitive than oral temperature in detecting ovulation, according to our peer reviewed paper published in the Journal of Medical Internet Research.

The study, which was the first to compare wrist skin temperature to BBT in detecting ovulation, found that Ava-WST had a higher true positive rate than BBT: Ava was able to identify ovulatory temperature shifts that BBT missed.

Typically, tests with a higher true positive rate (also called sensitivity) also have a higher false positive rate (resulting in lower specificity) and this was the case here: Ava-WST misidentified some ovulatory cycles that BBT did not misidentify. Ava-WST’s higher sensitivity and lower specificity makes it particularly well suited to women planning a pregnancy, since it was less likely to miss a true positive ovulation.

The Study: Wearable BBT vs Oral BBT

The prospective study included 193 cycles collected from 57 healthy women. Participants wore the Ava bracelet, a wearable BBT tracker, nightly. They also measured BBT orally and immediately upon waking using the Lady-Comp digital thermometer. An at-home luteinizing hormone test was used as the reference for ovulation.

The most important findings from the study:

• Ava-WST was more sensitive than BBT in detecting an ovulatory temperature shift (sensitivity 0.62 vs 0.23; P<.001)

• Ava-WST had a higher true-positive rate (54.9% vs 20.2%) for detecting ovulation; however, it also had a higher false-positive rate (8.8% vs 3.6%), resulting in lower specificity (0.26 vs 0.70; P=.002).*

• Ava-WST had a greater increase during the luteal phase compared with BBT (range of increase: 0.50 °C vs 0.20 °C). Ava-WST increased in a steeper and more continuous manner during the post-ovulatory/luteal phase

• Ava-WST had a greater and more rapid decrease during the menstrual phase (from 36.13 °C to 35.80 °C) than BBT (from 36.31 °C to 36.27 °C).

*While “true positive rate” and “sensitivity” are often used synonymously, this paper used slightly different definitions for the two terms. “True positive rate” in the paper refers to the proportion of correctly identified ovulatory cycles out of all cycles (ovulatory and anovulatory). “Sensitivity” refers to the proportion of correctly identified ovulatory cycles out of all ovulatory cycles. 

Tracking Fertility with Temperature

Human body temperature follows a circadian rhythm, meaning it varies over the course of a day, reaching a nocturnal minimum. In women with ovulatory menstrual cycles, this circadian temperature pattern is superimposed on a menstrual temperature pattern. Temperature increases during the post-ovulatory luteal phase compared with the pre-ovulatory follicular phase. In addition, temperature is modulated by lifestyle factors such as physical activity, meals, sleep, and ambient temperature.

As a practical matter, it can be challenging to disentangle circadian, lifestyle, and menstrual-related effects on temperature. As a daily point measurement, BBT curves are sensitive to missing values and the precise time of measurement; an ovulatory temperature shift may go undetected because a rise in body temperature may not have occurred yet at the time of measurement.

Wearable BBT versus Oral BBT: What’s the Difference?

There are a number of important differences between BBT and Ava-WST, some of which make Ava-WST better suited to detecting ovulatory temperature shifts:

The different modes of measurement as well as the different circadian rhythms of Ava-WST and BBT might explain the different performance of the two methods for identifying ovulation.

Oral BBT decreases continuously during sleep, reaching a nadir around 5AM, then rising sharply after waking. Point measurements such as BBT are susceptible to variations in waking times and compliance because a single measurement is located on the sliding scale of the circadian rhythm.

In contrast, the circadian rhythm of a wearable BBT tracker like Ava features a sharp increase before lights off, a plateau at a higher temperature during sleep, and then a sharp drop immediately after rising. Sleep propensity is accompanied by an increased skin blood flow and less cold-induced vasoconstriction, particularly in the distal skin areas that are most strongly involved in the regulation of heat loss, thus increasing the skin temperature. Once awake, cold-induced vasoconstriction is restored, thereby decreasing the skin temperature.

Ava-WST excludes the first 90 and last 30 minutes of recorded data, allowing it to capture the nocturnal steady state when temperature is maintained at a high level. This removed much of the effect of circadian rhythm and allowed the temperature changes to reflect mostly menstrual rhythm.

More Than Just Temperature

While this study looked at the performance of temperature in isolation from other parameters, in the real world, Ava-WST and BBT are integrated into more complex systems that increase their accuracy in detecting ovulation, as well as their overall utility for tracking fertility.

The wearable BBT was recorded by the Ava bracelet, which also measures heart rate, heart rate variability, breathing rate, and skin perfusion. This study looked only at the performance of temperature, but in the real world, the device makes fertility predictions based on a machine learning algorithm that incorporates multiple physiological parameters, improving its accuracy. The Ava bracelet was shown in prior research to detect the fertile window of the menstrual cycle with 90 percent accuracy.

Interpreting BBT alongside other fertility signs such as cervical mucus, cycle history, and LH tests can help users overcome ambiguity about the day of ovulation, improving the performance compared to relying on BBT alone.

For example, the symptothermal method of fertility tracking, which combines BBT measurement and cervical mucus observations, was evaluated as a contraceptive in one study. The study found that women who abstained from sex during the fertile window defined by the method had an unintended pregnancy rate of 0.43%.

Temperature is a lagging indicator of fertility; the observable rise in temperature occurs only after the fertile window is closed for the current menstrual cycle. Tracking leading indicators of fertility—LH and cervical mucus in the case of fertility awareness based method, or a multi-parameter machine learning algorithm in the case of the Ava bracelet—is necessary for real time detection of fertile days.

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.