The Food and Drug Administration (FDA) has approved the first mobile app that can be used as a method of birth control. The Natural Cycles app calculates the days of the month a woman is likely to be fertile based on body temperature readings and menstrual cycle information. 

The app works by using an algorithm centered on the user’s basal body temperature, which is entered into the app each day, and menstrual cycle information. Women using the app should abstain from sex or use protection (such as a condom) when they see ‘fertile day’ displayed on the app, typically 4 or 5 days a month. The algorithm was designed to account for sperm survival, variation in cycle length, ovulation day, temperature fluctuations and the length of the follicular and luteal phases of the menstrual cycle.

The algorithm identifies and displays ‘green’ or ‘red’ days, green for not fertile and red for fertile. The Company conducted clinical trials involving 15,570 women who used the app for an average of 8 months. The data showed a ‘perfect use’ failure rate of 1.8% (women who used the app for 1 year and become pregnant because they had sexual intercourse on a day when the app predicted they would not be fertile or because their contraception method failed), while the ‘typical use’ failure rate was 6.5% (women sometimes not using the app correctly, such as having unprotected intercourse on fertile days).

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“Consumers are increasingly using digital health technologies to inform their everyday health decisions, and this new app can provide an effective method of contraception if it’s used carefully and correctly,” said Terri Cornelison, MD, PhD, assistant director for the health of women in the FDA’s Center for Devices and Radiological Health. “But women should know that no form of contraception works perfectly, so an unplanned pregnancy could still result from correct usage of this device.”

The Natural Cycles website states that a 2 decimal basal thermometer is needed for use with the app; the Company provides a thermometer to individuals who sign up for an annual subscription. The annual subscription costs $79.99; monthly subscriptions are available as well for $9.99.

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This article originally appeared on MPR