April 1, 2024

Comparing Days of Fertility Using Different Apps: A Review of Research​

By: Sandra Spychalska, MD

Director’s Note: Today, we are highlighting research published by FACTS executive director, Dr. Marguerite Duane, and FACTS Board member, Dr. Mike Manhart. Their research compared the fertile windows identified by two popular fertility apps: CycleProGo and Natural Cycles. Dr. Sandra Spychalska, a family medicine resident and former FACTS elective participant, summarized this 2022 study [1] and discussed the limitations of the Natural Cycles app, especially when compared to fertility awareness-based methods (FABMs) that use multiple biomarkers of hormone activity. Last spring, we also published an interview on the benefits, burdens, and beauty of FABMs by Dr. Spychalska that explores one woman’s fertility journey and the learning curve across three different methods.

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Introduction

Fertility apps are commonly used to track observations related to hormone activity, including menstrual bleeding, cervical mucus, and basal body temperature (BBT). Many of these apps claim to predict a fertile window that may be used by women to either achieve or avoid pregnancy, but the way this fertile window is determined varies greatly from app to app. A 2022 study by Manhart and Duane compared the fertile windows identified by the apps CycleProGo and Natural Cycles.

“Many apps claim to predict a fertile window that may be used by women to either achieve or avoid pregnancy, but the way this fertile window is determined varies greatly from app to app.”

Natural Cycles is the first FDA-approved birth control app; with an unintended pregnancy rate of 8-9%, it relies on BBT to establish the fertile window. CycleProGo was developed by Couple-to-Couple League, and this app uses BBT and cervical mucus to determine the fertile window. The unintended pregnancy rate is approximately 2% in users of CycleProGo.

Methodology

Using data from the CycleProGo app, 20 women were randomly selected to have their first 12 charted cycles analyzed. To be included, participants had to be women younger than 40 years of age with a minimum of 12 charted cycles ranging from 20 to 40 days long. Data collected included cycle start and end dates, app-defined fertile window, daily BBT values, and observations of peak-type cervical mucus.

The BBT values from CycleProGo were entered into the Natural Cycles app to determine the start, end, and mean length of the fertile window. Data from CycleProGo were compared to data from Natural Cycles, and then compared to physiologically-determined fertile windows that use peak-type mucus to estimate ovulation.

Ovulation is thought to occur within 2 days of peak mucus in 88 to 95% of cycles. Thus, the physiologic narrow fertile window was defined as peak mucus minus 5 days through peak mucus (6 days total), while the wide fertile window was defined as the peak mucus minus 7 days through peak mucus plus 2 days (10 days total).

 “Ovulation is thought to occur within 2 days of peak mucus in 88 to 95% of cycles.”

Results

Using the inclusion criteria discussed above, 240 cycles from 20 women were used for data analysis. The mean age of participants was 29.5 years and the average cycle length was 28.3 days. BBT was recorded on 92% of days, and 186 cycles had sufficient cervical mucus observations to determine a peak day. Start and end dates of fertile windows identified by the two apps were determined to be equivalent if the dates were within 1 day of each other.

A fertile window start date was calculated by each app in 238 cycles, resulting in 57% of cycles having equivalent start days on both apps. In general, the fertile window was determined to start later with Natural Cycles compared to CycleProGo, with 36% of cycles starting 2+ days later using Natural Cycles. Compared to the narrow window start, Natural Cycles started 12% of cycles later, while CycleProGo started 4% of cycles later. Compared to narrow or wide physiology windows, Natural Cycles started 30% of cycles later, whereas CycleProGo started 18% of cycles later.

A fertile window end date was calculated by each app in 181 cycles, with 36% of cycles having equivalent end dates. The fertile window often ended earlier with Natural Cycles compared to CycleProGo, with 52% of cycles ending 2+ days earlier with Natural Cycles, and 34% ending 3+ days early. Although CycleProGo never ended the fertile window before the physiologic fertile window was closed, it only reported an end to the fertile window in 79% of cycles. On the other hand, Natural Cycles reported an end date in 92% of cycles but ended 13% of cycles before the end of the narrow physiologic window, and 38% of cycles before the end of the wide or narrow windows.

Image of young beautiful asian woman smiling and holding cellphone while working in office

“Natural Cycles reported an end date in 92% of cycles but ended 13% of cycles before the end of the narrow physiologic window, and 38% of cycles before the end of the wide or narrow windows.”

The average length of the fertile window in Natural Cycles was 12.8 +/- 3.64 days while in CycleProGo it was 15.1 +/- 3.53 days. In both apps, cycles 7 to 12 had shorter fertile windows compared to cycles 1 to 6 as the algorithms “learned” through the data provided.

Discussion

Despite an abundance of fertility apps on the market, little information is readily available to users about the algorithms used to determine a fertile window. Users of fertility apps benefit from learning the principles behind fertility awareness-based methods (FABMs) and their role in these algorithms. This study by Manhart and Duane demonstrated that even though BBT was used in both apps, the addition of cervical mucus observations led to different definitions of the fertile window. The lead author, Dr. Mike Manhart, added that “the difference can also be attributed to different algorithms used in each app. Since Natural Cycles does not disclose what their algorithm is, it is hard to know if these differences in fertile window are driven by the addition of mucus, differences in how BBT is used to define the fertile window, or both.”

In general, Natural Cycles started the fertile window later and ended it earlier than CycleProGo. Given these findings, and comparing these fertile windows to physiologic fertile windows, Natural Cycles may be missing important days of fertility at the start and end of the cycle. Even though Natural Cycles had a shorter mean length of the fertile window (12.8 days), this does not necessarily mean it is more accurate, especially when compared to fertile window lengths found through urinary hormones (14.3 days) and cervical mucus (14.2 days).[2] Also, the app showed a decrease in mean length in cycles 7 to 12 as the algorithm “learned,” but an increase in the number of fertile windows ending before the physiologic window closed, suggesting decreased length does not equate to increased accuracy in Natural Cycles. Based on these results, BBT alone may be less reliable to predict ovulation in shorter cycles, and the addition of cervical mucus findings with CycleProGo resulted in fewer cycles with missed days of fertility.

“In general, Natural Cycles started the fertile window later and ended it earlier than CycleProGo. Given these findings … Natural Cycles may be missing important days of fertility at the start and end of the cycle.”

Even though Natural Cycles is the first FDA-approved and one of the most popular birth control apps, this study suggests it may not be the most reliable in determining fertile windows, especially compared to methods that use multiple markers of hormone activity, such as cervical mucus and urinary hormones. The effectiveness and reliability of different FABM apps should continue to be studied to provide women with a variety of options that fit their lifestyle and goals.

References

[1] Manhart MD, Duane M. A comparison of app-defined fertile days from two fertility tracking apps using identical cycle data. Contraception. 2022;115:12-16. doi:10.1016/j.contraception.2022.07.007.
[2] Fehring RJ, Schneider M. Comparison of Abstinence and Coital Frequency Between 2 Natural Methods of Family Planning. J Midwifery Womens Health. 2014;59(5):528-532. doi:10.1111/jmwh.12216.

ABOUT THE AUTHOR

Sandra Spychalska, MD

Dr. Sandra Spychalska is a resident at Northwestern Lake Forest Family Medicine Residency. She is interested in holistic approaches to healthcare and is completing an integrative medicine certificate program. She enrolled in the FACTS elective to gain a better understanding of natural family planning methods and continues to use them in her practice. Dr. Spychalska enjoys educating women about a variety of methods for birth control and empowering them to learn about their bodies.

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