Natural Cycles, a contraceptive app that became certified in the EU as a form of birth control, has been hit with a complaint after being blamed for causing 37 unwanted pregnancies, reports Swedish agency SVT. Södersjukhuset hospital in Stockholm reported the app to Swedish regulator MPA (the Medical Product Agency), after 37 women visited the hospital for an abortion after becoming pregnant while using Natural Cycles.

The app uses an algorithm and measures factors like temperature to determine the period when a woman may be fertile. It’s a popular alternative to hormonal contraceptives like the pill because it lacks side effects. In a statement to The Verge, Natural Cycles said it hasn’t received any information from Södersjukhuset hospital, but it is in touch with the MPA about each individual case. The company has also initiated an internal investigation with their clinical department, and said:

“No contraception is 100% effective, and unwanted pregnancies is an unfortunate risk with any contraception. Natural Cycles has a Pearl Index of 7, which means it is 93% effective at typical use, which we also communicate. At first sight, the numbers mentioned in the media are not surprising given the popularity of the app and in line with our efficacy rates. As our user base increases, so will the amount of unintended pregnancies coming from Natural Cycles app users, which is an inevitable reality.”

The app currently has 700,000 users worldwide. A midwife at SÖS, Carina Montin, told SVT that the hospital was reporting Natural Cycles to the MPA because of the unwanted pregnancies it helped cause. The app is designed for users aged 18 years and over, and Natural Cycles says younger users should consider another form of contraception.