The app costs £6.99 per month, has around 100,000 UK users and works on a fairly simple principal. During ovulation, higher progesterone levels increase a woman's body temperature nearly a half degree Celcius (0.8 Fahrenheit).

Users measure their temperature daily using the included electronic thermometer, and the app compares the readings with its dataset. Days when unprotected sex is okay are marked as green, but if the calendar shows red, couples must use condoms, the pill or other protection. It can also be used in the opposite way, telling couples the optimal time to have sex if they want a child.

"We can now proudly say that Natural Cycles is an effective contraceptive, comparable with others like the pill, the IUD and condoms," Natural Cycles co-founder Elina Berglund wrote on the company's site. "It's another option for women to choose from."

It wasn't quite a straight line to get there, though. In 2015, the company was barred from advertising the app as a contraceptive in its home country of Sweden. That pushed it to hire a team of researchers, who wrote a paper arguing it was nearly as effective as other methods. Last year, it turned that information over, along with all its company processes to Tüv Süd, the German firm that certified it. It should be worth the effort, though, as doctors across Europe can now recommend it.

It's not for everyone, experts caution. "Any device that monitors the menstrual cycle is fallible as women don't always ovulate predictably," Dr. Adam Balen told The Telegraph. The UK's National Health Service also points out that similar methods result in seven pregnancies per 100 women in a year (comparable to a condom), instead of one in 100 for techniques like the pill, implant or IUD.

Because of that, the company recommends it most for stable couples or women who have trouble with other contraception methods. "Our product is ideally suited for women in a relationship as well as women who feel bad from their currently used contraceptive," co-founder Raoul Scherwitzl tells Wired.