Contraceptives are never a sure thing as the medical community has noted over the years. However, one contraceptive app in particular – Natural Cycles – has attracted a bit of controversy after its use reportedly resulted in 37 women unintentionally becoming pregnant.

The news comes from Swedish hospital Södersjukhuset which obtained the information via a study performed on 668 women - all of whom came to the hospital seeking abortions. The hospital claims they had a "duty to report all side effects, such as pregnancies, to the Medical Products Agency."

For the unaware, Natural Cycles allows women to more easily track their menstrual cycles. The app takes into account the user's body temperature to determine when it will be safe to have intercourse without other forms of contraception such as condoms or birth control pills. "Safe days" are marked in green while days that the app deems unsafe are marked in red. As of writing, the app is currently used by over 500,000 women globally.

While it may seem risky to rely on an app alone for contraception, it's not blind trust that has led to its widespread adoption. Not only has Natural Cycles proven to be an effective form of contraception in the company's own studies, German regulatory body Tuv Sud has also deemed it an official form of contraception according to Engadget.

That said, with so many women using Natural Cycles globally, it seems inevitable that a few might slip through the cracks. Natural Cycles concurs, stating that "No contraception is 100 percent and unwanted pregnancies [are] an unfortunate risk with any contraception." The company later issued a second statement, noting that their app has a "Pearl Index of 7, which means it is 93 percent effective at typical use."