What is the app?
According to its website, the app, certified as a form of contraception by the European Union, calculates which days a woman will be fertile by taking factors like her temperature and menstrual cycle into account.
On days it calculates as fertile days, the app alerts the user to either abstain from sex or use an alternate form of contraception such as a condom.
The company is headquartered in Stockholm, and was founded by married physicists who initially developed the app for their own use.
The app is billed as a non-hormonal, non-invasive alternative to methods like the birth control pill or the IUD.
According to Business Insider, Swedish news outlet SVT reported that a hospital in the country reported the app to a local medical device regulator after finding 37 of its 668 patients who underwent an abortion between September and December 2017 said they used Natural Cycles as contraception.
Carina Montin, a midwife at the Stockholm hospital, told SVT, "It's a new method and we see a number of unwanted pregnancies, so we are reporting this to the Medical Products Agency."
The hospital did not say whether the patients in question were using the app correctly when they became pregnant.
What did the company say?
A spokesman for Natural Cycles told Business Insider that the app’s effectiveness is comparable to other types of contraception, and no birth control method is "100% effective."
"Natural Cycles has a Pearl Index of 7, which means it is 93% effective at typical use, which we also communicate," the spokesman said. "Our studies have repeatedly shown that our app provides a high level of effectiveness similar to other methods."
The company said it is in touch with medical regulators and launched an internal investigation.
"As our user base increases, so will the amount of unintended pregnancies coming from Natural Cycles app users, which is an inevitable reality," the spokesman added.