A birth control cellphone app has been certified as a medical device in Europe, but doctors are warning users that it should not be used as a solo source of contraception. 

The app, Natural Cycles, is available in Apple's App Store and the Google Play store to users everywhere. It digitizes an age-old method of preventing pregnancy, sometimes called the rhythm method, natural family planning or fertility awareness. 

The idea is to track ovulation and avoid sex (or use additional protection) on days when a woman is most likely to be fertile. These methods have a failure rate of about 25 percent, even though the promotional materials for Natural Cycles claim that the app is 93 percent effective. 

Dr. Carol Dunetz, of NYU Langone Gynecology Associates in Lake Success, says she is concerned that the app is being marketed as a contraception method. 

"There are a lot of variables to consider," she says. "To get the success rate they're claiming, you have to disregard all these variables," says Dunetz.

The company that makes Natural Cycles is headquartered in Sweden. The app was certified as a medical device in February, making it the only app that can call itself "contraception" in Europe. 

The app tracks body temperature and the menstrual cycle – taking into account the amount of time that sperm are likely to survive in the body. 

There are more than 250 versions of menstruation apps available.