Continuous wrist skin temperature monitoring precisely detects fertile window

Continuous monitoring skin temperature, through a wrist-worn device that records measurement during sleep, is more sensitive than basal body temperature (BBT) at detecting ovulation, according to a recent study. 

“This study is the first to compare the wrist skin temperature and BBT in detecting ovulation,” said the researchers, pointing out that “continuously measured wrist skin temperature is more sensitive than BBT to detect ovulation and determine the fertile window.”

Fifty-seven healthy women (mean age 26.7±4.2 years) participated in the study and were asked to wear a fertility tracker device on their wrists to continuously measure skin temperature during sleep. BBT, as a comparator indicator, was measured daily upon waking up using a digital oral thermometer. Ovulation was detected using an at-home luteinizing hormone (LH) test.

Overall, 193 cycles were available for analysis, 88.1 percent of which were ovulatory, as determined by a positive LH test. Relative to BBT, wrist monitoring detected a significantly higher percentage of ovulatory cycles with at least one temperature shift (62.4 percent vs 22.9 percent; p<0.001). Such a shift, however, was detected almost 2 days earlier on the BBT curves (p<0.001). [J Med Internet Res 2021;23:e20710]

Using the LH test as reference, wrist temperatures proved to be significantly more sensitive than BBT monitoring (0.62 vs 0.23; p<0.001) and provided a greater true-positive rate (54.9 percent vs 20.2 percent). However, its false positive rate was also higher (8.8 percent vs 3.6 percent), which led to a significantly lower specificity (0.26 vs 0.70; p=0.002).

The wrist device likewise demonstrated a slightly higher positive predictive value. For each temperature shift detected, there was an 86.2-percent probability of ovulation. When using BBT, the probability was 84.8 percent. Negative predictive values of both methods were low and statistically comparable (p=0.39).

Both basal body and wrist temperatures were significantly correlated with each other, but only during the follicular phase (p=0.001). Both measures also increased during the postovulatory phase, though the magnitude of change was higher for wrist temperature. During the menstrual phase, temperature measurements dropped but with greater magnitude and speed as measured by the wrist device.

“For women interested in maximizing the chances of pregnancy, wrist skin temperature continuously measured during sleep is more sensitive than BBT for detecting ovulation,” said the researchers. “The difference in the diagnostic accuracy of these methods was likely attributed to the greater temperature increase in the postovulatory phase and greater temperature decrease during the menstrual phase for the wrist skin temperatures.”

“However, when used as a standalone method, neither of the temperatures could reliably avoid unplanned pregnancy, given the low negative predictive values,” they said. “Our results underpin the importance of validation studies, especially against a standard reference test, while developing wearable devices that measure physiological parameters for women or clinicians to track menstrual cycles.”