After 37 women who allegedly relied on the European Union-certified fertility app Natural Cycles as a non-hormonal method of birth control ended up with unwanted pregnancies in the last quarter of 2017, according to the Swedish news agency SVT, the U.S. Food and Drug Administration (FDA) approved the marketing of this app as a method of contraception, according to an August 10 press release.

The app, which is available on Google Play and Apple's App store, has more than 500,000 users in 161 countries, according to data from Natural Cycles. Its technology relies on inputing daily body-temperature readings from a thermometer to predict a user's ovulation, since body temperature rises slightly when your ovaries release an egg. Its algorithm also accounts for cycle irregularities and sperm survival rates, while an optional ovulation test detects the hormones released one to two days before ovulation.

In theory, if you avoid unprotected sex for about five days before ovulation and about two days after, you can avoid getting pregnant, according to Planned Parenthood. The healthcare provider estimates tracking fertility using body temperature, changes in cervical mucus, or by estimating dates of ovulation based on when you're expecting your period (i.e., Fertility Awareness Methods or FAMs) can prevent pregnancy 76 to 88 percent of the time. By comparison, when used perfectly, which isn't always realistic IRL, condoms are 98 percent effective; IUDs are 99 percent effective; and birth control pills are 99 percent effective, according to Planned Parenthood.

The makers of Natural Cycles claim the app prevents unwanted pregnancy just 93 percent of the time, which could be why 37 reported app users got pregnant and sought an abortion between September and December 2017 at Södersjukhuset Hospital in Stockholm, Sweden, according to SVT.

"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, M.D., Ph.D., assistant director for the health of women in the FDA’s Center for Devices and Radiological Health in a statement. "But women should know that no form of contraception works perfectly, so an unplanned pregnancy could still result from correct usage of this device."

In a January 2018 statement, a Natural Cycles spokesperson expressed the same sentiment: "No contraception is 100 percent effective, and unwanted pregnancies is an unfortunate risk with any contraception... As our user base increases, so will the amount of unintended pregnancies coming from Natural Cycles app users, which is an inevitable reality."

To increase the effectiveness of any FAM, Planned Parenthood recommends combing methods by, for instance, monitoring your cervical mucus in addition to using a temperature-tracking app such as Natural Cycles. At the end of the day, FAMs still don't work as well as other types of birth control—something to keep in mind when deciding how to best protect yourself.

Update 8/14: This post has been updated to reflect the FDA's approval of Natural Cycles.

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Elizabeth Narins
Senior fitness and health editor

Elizabeth Narins is a Brooklyn, NY-based writer and a former senior editor at Cosmopolitan.com, where she wrote about fitness, health, and more. Follow her at @ejnarins.