There are several apps that promise to track periods, and presumably fertility, but only one in the US is permitted to actually advertise itself as birth control.
Natural Cycles uses a woman’s basal body temperatures, or the lowest temperature the body reaches, which occurs first thing in the morning, to determine her fertility. Since a woman’s body temperature increases slightly just after she releases an egg, every time she enters her basal body temperature into Natural Cycles, the app gains data as to where she may be in her cycle. (You get a free thermometer with a $79.99 annual subscription to Natural Cycles to take those measurements.) This algorithm can eventually predict when users are most fertile, and should use a back-up form of birth control, if they are trying to avoid pregnancy, or try to conceive, if that’s the goal. It’s an automated form of “fertility awareness.”
For the first two to four months while the app gathers temperature data, women need to use backup forms of birth control, Juan Acuna, a gynecologist at the University of Florida and a scientific advisor for Natural Cycles, told STAT. After that the failure rates are similar to other forms of birth control methods.
On Friday, Aug. 10, the US Food and Drug Administration gave the go-ahead to Natural Cycles to market itself as a method of contraception. That decision came after the agency reviewed a clinical studies of the app that included nearly 16,000 women using it for eight months. Women who used the app perfectly—meaning, they took their temperatures properly and heeded the app’s advice—became pregnant 1.8% of the time (a number referred to as a “failure rate” in contraception research). The same study found that when used “typically,” Natural Cycle’s failure rate was around 6.5%. For comparison, the birth control pill has a typical-use failure rate of 9%, and condoms are at 18%. That said, other forms of birth control have much more data and long-term research backing up those numbers.
Earlier this year, Natural Cycles received negative attention when a hospital in Sweden reported that 37 out of 668 women seeking abortions had become pregnant while using the app to as birth control. (It had already been approved as a form of birth control in Europe.) The app’s creators weren’t deterred by these numbers, saying that Natural Cycles has a typical-use problem just like many other methods of birth control.
Still, that doesn’t mean this app is suitable for all women trying to avoid pregnancy. As Quartz’s Rosie Spinks has pointed out:
It is undeniably hard work, especially when compared to other forms of birth control. The basal body temperature tracking method assumes several things: You have a stable enough life to be able to wake up at roughly the same time every single day (and will remember to do a temperature reading before getting up to pee, cuddling your partner, or even sitting upright in bed). You don’t travel, have hangovers, stay up late working, sleep in, or get stressed too often—all of which can affect your temperature reading that day and when you ovulate that month, and thus render the algorithm less accurate. You and your partner are willing to commit to either abstaining from sex when you’re fertile—often when a woman’s libido is highest—or using a barrier method. And on top of all of that, you need to have total agency over whether a man has sex with you and when.