Drawing on clinical studies involving 15,570 women, the FDA states that the app's efficacy rate is 93.5 percent for typical use and 98.2 percent for perfect use.

Natural Cycles uses its own basal body thermometer and algorithm to calculate when a woman is fertile or infertile. It requires that users log their menstrual cycles and take their temperature at the same time every morning, before getting out of bed.

In doing so, the FDA created a new category for birth control called "software application for contraception."

The Food and Drug Administration recently approved mobile fertility app Natural Cycles as a method of contraception. While Natural Cycles was certified in Europe last year, this is the first time the U.S. has allowed a mobile app to advertise itself as a medical device that prevents pregnancy.

In January, the app came under fire when a Stockholm hospital reported that 37 women seeking abortions were using Natural Cycles as birth control. But the Swedish Medical Products Agency concluded in September that the unwanted pregnancies were inline with typical use failure rates.

Still, the discipline involved with the daily tracking of one's cycle and temperature has some doctors concerned.

After all, while using an algorithm may make Natural Cycles seem high-tech, the premise behind the app is nearly a century old. Birth control methods based on tracking ovulation are called fertility-awareness methods, and they've been around since the 1930s. According to the Centers for Disease Control and Prevention, these methods are only 76 percent effective. The most well known is the rhythm method, in which women count the days in their menstruation cycle to predict fertility. When women combine counting days with tracking other signs, like temperature or cervical fluid appearance, clinical trials demonstrate much higher efficacy rates.

Doctors and researchers have been eager to see more randomized control trials of the app. And some have criticized past studies of Natural Cycles for being run by the company's founders and advisory board members instead of neutral parties.

Yet experts expect the prevalence of app-based family planning will continue to rise. Whether women are using fertility-awareness methods for religious reasons or to avoid hormones or implants, there are already nearly 100 apps designed to track menstrual cycles. Glow, Kindara, Daysy and Groove already offer fertility tracking options for women seeking to avoid pregnancy.

The door is now open for these apps to seek FDA approval and even promote themselves as contraception.