By Vivian Chi
Editor’s Note: Last week we published the summary of a narrative review of wearable technology and personal devices currently available to track a woman’s fertile window. The authors mentioned the Oura ring as an example of a wearable that tracks health parameters. Below is the review of a 2019 study by Maijala et al titled, “Nocturnal Finger Skin Temperature in Menstrual Cycle Tracking: Ambulatory Pilot Study Using a Wearable Oura Ring.” [5] Vivian Chi summarized the research article while on the FACTS online elective in fertility awareness. The results of this small pilot study (n=22) suggest the Oura ring could be a useful tool to monitor the menstrual phase; however, some of the study limitations are discussed below, including the need for larger studies to validate the applicability of this practical wearable in real life.
Introduction
In the age of smartphones, apps, and wearable tech, the healthcare industry has been inundated with a myriad of new methods to track the menstrual cycle. Tracking one’s cycle is crucial to identify the fertile window, recognize symptoms, and monitor one’s health as a vital sign. Basal body temperature (BBT) has been used to track the fertile window—based on the temperature shift before and after ovulation—and to predict the onset and length of the ovarian-menstrual cycle. [1] [2] Previous studies have utilized intermittent rectal, oral, abdominal, and wrist skin temperature measurements [2] [3] [4] as part of a controlled experiment or as wearable technology, such as a wristband. Comfortable and compact, the Oura ring provides a unique style of wearable technology that can sense the patient’s temperature continuously and also track sleep and physical activity. This study by Maijala et al had two goals: (1) to assess the applicability of the Oura ring to monitor menstrual cycles and (2) to develop an algorithm to predict the start of menstruation and ovulation. [5]
“Tracking one’s cycle is crucial to identify the fertile window, recognize symptoms, and monitor one’s health as a vital sign.”
Methodology
Although 31 women volunteered for the pilot study, 9 were excluded or dropped out; thus, a total of 22 women participated in the study. Each participant received the Oura ring and information about study procedures, including opening the Oura app every morning to upload data and proper use of the ring. The study collected data for 120-150 days between late spring and early autumn in Finland. Using data from a previous pilot study that measured skin temperatures with the Oura ring, the researchers utilized a MATLAB script that would filter out temperature fluctuations and calculate the highest stable nocturnal temperatures. Other MATLAB algorithms used included formulas to predict menstruation and ovulation dates based on days with minimum and maximum temperatures. Participants were also instructed to take oral body temperatures immediately after waking up for at least one ovarian-menstrual cycle length and use a urine luteinizing hormone (LH) test to check for ovulation days to compare with the Oura ring results. Statistical analyses were performed to compare oral and finger skin temperatures by using repeated measures correlation for daily temperature values and dependent t-test for mean luteal phase and follicular phase temperatures.
Results
Skin and oral temperatures were moderately correlated with a repeated measures correlation coefficient of 0.563 (p<0.001), validating the use of the ring as a means of accurate temperature measurement. Results from the participant’s data confirmed a difference in body temperature between the follicular and luteal phases by 0.3 °C for skin and 0.23 °C for oral temperature (p<0.001 for both measurements) and a repeated measure correlation coefficient of 0.589 (p<0.004). Algorithm testing revealed a sensitivity of menstruation prediction of 81.4% and 86.5% with window lengths of ±3 and ±4 days, respectively. Ovulation was detected 95% of the time ±4 days from verified ovulation.
Discussion
This study confirms differences in skin temperature could be evaluated with the Oura ring and extrapolated to track the female cycle, but with some variability in the data. Menstruation prediction dates were only accurate within a larger window of time. For example, menstruation was detected with a sensitivity of 86.5% for ±4 days vs. 81.4% for ±3 days. Ovulation was detected with a sensitivity of 83.3% with fertile window detection from -3 to +2 days, which corresponds to previous studies that have used other forms of wearable sensors. [4] [6] [7] Based on these results and given the study limitations, more research is needed to determine whether the Oura ring could be used to accurately predict menstruation and/or ovulation.
“This study confirms differences in skin temperature could be evaluated with the Oura ring and extrapolated to track the female cycle, but with some variability in the data.”
Study limitations include the small sample size and high variability in the women’s health characteristics. In addition, the Oura ring temperature sensors were not calibrated prior to the study and oral daily temperature measurements were not necessarily taken at the same time of day for all participants. The results from this study must be validated with a larger sample size to have generalizability. Furthermore, the fact that the health tech company that makes the Oura ring provided the funding and devices for this study may contribute to biased results.
Being able to track one’s cycle with a small wearable such as the Oura ring could be convenient, reassuring, and empowering, but perhaps at the cost of accessibility, privacy concerns, and its price. Not all patients who are interested in temperature monitoring may have access to a wearable device, let alone one as expensive as the Oura ring. Those who are concerned about data privacy may not opt for this tracking method. For those who have access to the devices and are interested in tracking their cycle with ease, the Oura ring could be beneficial if further studies validate its use for this purpose.
“Being able to track one’s cycle with a small wearable such as the Oura ring could be convenient, reassuring, and empowering, but perhaps at the cost of accessibility, privacy concerns, and its price.”
References
[1] Fukaya K, Kawamori A, Osada Y, Kitazawa M, Ishiguro M. The forecasting of menstruation based on a state-space modeling of basal body temperature time series. Stat Med. 2017;36(21):3361–79.
[2] Chen W, Kitazawa M, Togawa T. Estimation of the biphasic property in a female’s menstrual cycle from cutaneous temperature measured during sleep. Ann Biomed Eng. 2009;37(9):1827.
[3] Mazerolle SM, Ganio MS, Casa DJ, Vingren J, Klau J. Is oral temperature an accurate measurement of deep body temperature? A systematic review. J Athl Train. 2011;46(5):566–73.
[4] Shilaih M, Goodale B, Falco L, Kübler F, De Clerck V, Leeners B. Modern fertility awareness methods: wrist wearables capture the changes in temperature associated with the menstrual cycle. Biosci Rep. 2018;38(6). https://doi.org/10.1042/BSR20171279
[5] Maijala, Anna & Kinnunen, Hannu & Koskimäki, Heli & Jämsä, Timo & Kangas, Maarit. (2019). Nocturnal finger skin temperature in menstrual cycle tracking: ambulatory pilot study using a wearable Oura ring. BMC Women’s Health. 19. 10.1186/s12905-019-0844-9
[6] Luo L, She X, Cao J, Zhang Y, Li Y and Song PXK. Detection and Prediction of Ovulation from Body Temperature Measured by An In-Ear Wearable Thermometer. IEEE Trans Biomed Eng. 2019:[Epub ahead of print] doi: https://doi.org/10.1109/TBME.2019.2916823
[7] Goodale BM, Shilaih M, Falco L, Dammeier F, Hamvas G, Leeners B. Wearable sensors reveal Menses-driven changes in physiology and enable prediction of the fertile window: observational study. J Med Internet Res. 2019;21(4):e13404.
ABOUT THE AUTHOR
Vivian Chi
Vivian Chi is a fourth-year osteopathic medical student attending Touro University Nevada in Henderson, Nevada. She earned her undergraduate degree at University of California Irvine and completed a one-year post-baccalaureate program at Cal State LA. She plans to pursue residency in family medicine with a particular clinical interest in women’s health. She enrolled in the FACTS elective to gain the tools needed to be a well-rounded primary care physician.
