A total of 37 women who were using a fertility app as a form of birth control had unwanted pregnancies in the last quarter of 2017.
The app, Natural Cycles, is used by more than 500,000 users across 161 countries, according to the company's data.
It uses technology that involves users taking daily body temperatures with a thermometer, which are used to predict when the person is ovulating, and therefore monitoring which days you can become pregnant.
However, Södersjukhuset Hospital in Stockholm, Sweden, found that a number of people who fell pregnant unexpectedly were using the app as a form of contraception, according to Cosmopolitan.
Of the 668 women who sought an abortion at the hospital between September and December 2017, 37 of them were relying on Natural Cycles.
In a statement, a spokesperson for the app said: "No contraception is 100 percent effective, and unwanted pregnancies is an unfortunate risk with any contraception."
"As our user base increases, so will the amount of unintended pregnancies coming from Natural Cycles app users, which is an inevitable reality."
The spokesperson added: "The numbers mentioned in the media are not surprising given the popularity of the app and are in line with our efficacy rates."
"We have initiated an internal investigation with our clinical department in order to confirm this."