Perfect-use and typical-use Pearl Index of a contraceptive mobile app

The Natural Cycles application is a fertility awareness-based contraceptive method that uses dates of menstruation and basal body temperature to inform couples whether protected intercourse is needed to prevent pregnancies. Our purpose with this study is to investigate the contraceptive efficacy of the mobile application by evaluating the perfect- and typical-use Pearl Index.

In this prospective observational study, 22,785 users of the application logged a total of 18,548 woman-years of data into the application. We used these data to calculate typical- and perfect-use Pearl Indexes, as well as 13-cycle pregnancy rates using life-table analysis.

We found a typical-use Pearl Index of 6.9 pregnancies per 100 woman-years [95% confidence interval (CI): 6.5-7.2], corrected to 6.8 (95% CI: 6.4-7.2) when truncating users after 12months. We estimated a 13-cycle typical-use failure rate of 8.3% (95% CI: 7.8-8.9). We found that the perfect-use Pearl Index was 1.0 pregnancy per 100 woman-years (95% CI: 0.5-1.5). Finally, we estimated that the rate of pregnancies from cycles where the application erroneously flagged a fertile day as infertile was 0.5 (95% CI: 0.4-0.7) per 100 woman-years. We estimated a discontinuation rate over 12months of 54%.

This study shows that the efficacy of a contraceptive mobile application is higher than usually reported for traditional fertility awareness-based methods. The application may contribute to reducing the unmet need for contraception.

The measured typical- and perfect-use efficacies of the mobile application Natural Cycles are important parameters for women considering their contraceptive options as well as for the clinicians advising them. The large available data set in this paper allows for future studies on acceptability, for example, by studying the efficacy for different cohorts and geographic regions.