February 5, 2020
By Anastasia G. Xenophontos

Editor’s Note: This is a review of research[i] published in 2018 by Shilaih et al in Bioscience Reports titled, “Modern fertility awareness methods: wrist wearables capture the changes in temperature associated with the menstrual cycle.” It was written by a medical student as part of a two-week online elective taught by FACTS executive director, Dr. Marguerite Duane, at Georgetown University School of Medicine.

Basal body temperature (BBT) is an important fertility sign used in many fertility awareness based methods (FABMs). Basal body temperature shifts around the middle of the menstrual cycle, indicating ovulation and reflecting changes in estrogen and progesterone levels. This shift is known to create a biphasic temperature pattern in which temperatures remain low from the beginning of menstruation up until ovulation (reflecting low progesterone levels) and then rise once ovulation has occurred until the next menstruation (reflecting increased progesterone levels). This temperature shift indicates ovulation has already occurred and allows women to retrospectively identify the end of their physiologic fertile window (occurring about 24 hours after ovulation). The lowest temperature in the cycle (known as the nadir) corresponds to a peak in estrogen and occurs just prior to ovulation, thus potentially serving as a marker of approaching ovulation.

Due to the low cost and ease of taking one’s temperature at home, BBT is a popular fertility tracking method. BBT monitoring involves taking a morning rectal, oral or vaginal temperature immediately upon waking and charting these temperatures daily throughout the menstrual cycle. Women using BBT as a means of natural family planning often follow the ‘three-over-six’ rule. According to this rule, a temperature shift has occurred when a woman records three consecutive days of temperatures that are at least 0.2◦C higher (an increase by 0.4◦F) than the temperatures of the six preceding days. This change indicates the start of the infertile phase.

Despite being cost-effective, the BBT method carries a potential for errors in measurement (i.e., using an insensitive thermometer or taking temperatures at inconsistent times) and misinterpretation of charts, both of which would reduce the method’s efficacy. Furthermore, research has shown BBT readings can be affected by a variety of factors, such as having sexual intercourse or consuming coffee or alcohol before sleep, thus interfering with accurate interpretation of temperature changes.[ii] Additionally, some women find it difficult to sustain the process of waking up at the same time each day to diligently chart their temperatures.

This study sought to determine whether tracking wrist skin temperature (WST) automatically during sleep by wearing a recording device could serve as a sustainable and easier alternative to traditional BBT monitoring.

This observational study took place in Switzerland and included 194 women ages 20 to 40 years old with regular menstrual cycles (24-36 days). The participants were recruited from the Department of Reproductive Endocrinology at the University Hospital Zurich. Wrist skin temperature was recorded for a total of 793 cycles. After exclusions to meet study parameters, 437 menstrual cycles of 136 participants were recorded and analyzed.

• WST Tracking
Participants were asked to measure their temperature each night as they slept using the Ava bracelet, a wearable device that measures skin temperature from the wrist. The device measures temperature every 10 seconds, providing continuous WST measurements during sleep. For each menstrual cycle recorded, the study identified a temperature shift, if present, and the temperature nadir. Temperature shifts were defined in accordance with the three-over-six rule. The average WST during each menstrual phase was also recorded for each cycle.

Each phase of the cycle was clearly defined: the menstrual phase (first day of menses to 5 days later), the follicular phase (first day post-menses through ovulation day [OV -6]), the fertile phase (OV -5 through OV), the early-luteal phase (OV +1 through OV +7), and the late-luteal phase (OV +8 through the day prior to the start of menses).

• LH Urine Test
To determine the day of ovulation, women were asked to perform an at-home morning urine LH test. This has been shown to correspond with ultrasound determination of ovulation with 90-100% accuracy.[iii] The women were asked to take the test each morning beginning 5 days after the start of menses and extending through confirmed ovulation.

• BBT Documented Covariates
To control for potential confounding factors previously shown to influence BBT measurements, participants tracked certain activities electronically each day. Their diaries indicated whether they ate, drank coffee or alcohol, engaged in sexual intercourse, or exercised within 3 hours before sleep. Throughout the study, they also documented any bleeding taking place outside of the menstrual phase as spotting.

Statistical Analysis
The association between WST and menstrual phase was analyzed using “linear mixed effects models with random intercepts and random slopes.” This facilitated the interpretation of repeated measurements. The effect of each potential covariate on WST throughout the cycle was measured using similar models. A single multivariate, multilevel model including significant covariates was run to determine the association between WST and menstrual phases while controlling for significant covariates.

In the final sample, the average number of cycles recorded per participant was 3.21, with the average cycle lasting 28.84 days. Using WST, the study detected a temperature shift in 82% of cycles. Most temperature shifts (86%) occurred on or after ovulation, indicating WST is a reliable retrospective marker of ovulation. In most cases, the temperature nadir occurred prior to ovulation, but it was only detected in the fertile window in 41% of the cycles. Thus, the nadir is not a reliable prospective sign of ovulation using WST tracking

Average WST across the menstrual phases replicated traditional BBT tracking patterns, with the average WST during the menstrual phase being significantly lower than the average WST during the early-luteal phase and late-luteal phases, and the average WST during the fertile phase being significantly lower than that of the menstrual phase

These results remained significant even after controlling for covariates. A higher BMI, having sexual intercourse before bed, and eating a large meal before sleep significantly increased nightly WST. However, these covariates did not significantly affect the detectability of temperature shifts or menstrual phase effects on WST described above.

This study found that WST tracking replicates the detectability of temperature shifts and menstrual phase effects on temperature traditionally identified with BBT tracking methods. Furthermore, WST appears to be robust to confounding factors that influence traditional BBT readings. Thus, measuring nightly WST using a wearable device may serve as a more convenient and environmentally stable alternative to traditional BBT methods for fertility tracking, especially if used in conjunction with biological markers denoting the start of the fertile window and/or ovulation.

Editor’s Note: The articles used as references for this research review may be of interest to our readers. “Fertility awareness-based methods: another option for family planning” was published in the Journal of the American Board of Family Medicine in 2009. It provides a thorough summary of several cervical mucus-based methods and an exhortation for physicians to learn more about these effective, modern evidence-based FABMs. The article by Behre et al discusses the use of a fertility monitor as a tool to help women time intercourse when seeking to conceive, and as an aid to monitor treatment of infertility.



[i] Shilaih, M., Goodale, B.M., Falco, L., Kubler, F., De Clerck, V., Leeners, B. (2018) Modern fertility awareness methods: wrist wearables capture the changes in temperature associated with the menstrual cycle. Bioscience Reports. 38, https://doi.org/10.1042/BSR20171279.
[ii] Pallone, S.R. and Bergus, G.R. (2009) Fertility awareness-based methods: another option for family planning. J. Am. Board Fam. Med. 22, 147–157, https://doi.org/10.3122/jabfm.2009.02.080038.
[iii] Behre, H.M., Kuhlage, J., Gassner, C., Sonntag, B., Schem, C., Schneider, H.P. et al. (2000) Prediction of ovulation by urinary hormone measurements with the home use ClearPlan Fertility Monitor: comparison with transvaginal ultrasound scans and serum hormone measurements. Hum. Reprod. 15, 2478–2482, https://doi.org/10.1093/humrep/15.12.2478.

Author Bio: Anastasia G. Xenophontos is a medical student at Georgetown University School of Medicine. Anastasia earned her Bachelor of Science in Biopsychology, Cognition, and Neuroscience in 2016 from the University of Michigan and was a Post-Baccalaureate Research Fellow in the Developmental Neurogenomics Unit of the NIH from 2016 to 2018. While at the NIH, Anastasia conducted research on sex chromosome dosage effects on cortical morphology. She recently completed the FACTS elective.

Our NEW 2020
Webinar Series!

Beyond our core webinars, FACTS is pleased to present a series of new webinars this year — one offered each month through June 2020! These bonus presentations reflect current research and topics of growing interest in the field of fertility awareness.

Join us!

Registration is now OPEN for these 1-hour webinars. To REGISTER, click on the dates below.

Data-Powered Women’s Health: Phenotyping Endometriosis  |  Tuesday, February 25th, 8:30 pm EST
Evaluating Female Interest for Health Monitoring and Family Planning  |  Tuesday, March 24th, 8:30 pm EST
What Can Be Learned from Millions of App Records  |  Tuesday, April 28th, 8:30 pm EST
What’s New with Fertility Tracking Apps?  |  Tuesday, May 26th, 8:30 pm EST
Chart Neo & the Visual Fertility Advisor  |  Tuesday, June 23rd, 8:30 pm EST


FACTS members can attend one FREE webinar each year.

We hope you will join us!

To become a FACTS member, click here
For more information, please email our webinar coordinator at speaker@FACTSaboutFertility.org.


Join Us for Our Core Webinars and Earn CME Credit!

Below are the webinars we offer every month.
To REGISTER, click on the dates below, then share and invite a colleague!

The Female Cycle as the 5th Vital Sign
This presentation highlights the health of the female cycle and how the fertility awareness chart can aid in the diagnosis and management of common women’s health concerns.
Wednesday, February 19th, 6:30 pm EST
Wednesday, March 18th, 12:30 pm EST
Tuesday, April 7th, 8:00 pm EST


FABMs for Achieving & Avoiding Pregnancy
This presentation focuses on the use of FABMs to achieve pregnancy. It includes a case in which FABM charts assisted to diagnose and treat infertility. Evidence-based FABMs and their effectiveness rates to prevent pregnancy are also discussed.
Monday, March 9th, 7:00pm EST
Monday, April 27th, 1:00pm EST


The Signs of the Female Cycle Explained
Designed for a general audience, topics of this presentation include the science of charting a woman’s cycle, the benefits of using FABMs to monitor health, and how to choose a method that may be best for you!
Friday, February 14th, 1:00 pm EST
Monday, March 2nd, 12:00 pm EST


The Science Behind Fertility Awareness
Our flagship presentation, Fertility & Family Planning, reviews the latest research and science supporting FABMs and their effectiveness to prevent and achieve pregnancy.
Monday, March 30th, 5:00 pm EST
Thursday, May 21st, 1:00 pm EST

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