A birth control app has reportedly been referred to authorities in Sweden, after a hospital discovered a number of cases of unwanted pregnancies among women who relied on the app as a method of contraception.

Natural Cycles relies on the traditional ‘rhythm method’ to predict the chances of a woman getting pregnant. Since last year, it is the first app to be certified as a contraceptive in the EU.

Every morning, users take their temperature by placing a thermometer under their tongue, before recording the result in the app. Natural Cycles then uses an algorithm to calculate the users’ menstrual cycle and informs them when they can have sex without protection.

The company, which is based in Stockholm, Sweden, claims the app is 93% effective and is comparable to typical use of the contraceptive pill.

Yet its reliability has been brought into dispute after claims that 37 out of 668 women who sought an abortion at one of Stockholm’s largest hospitals from September to December 2017 were relying on Natural Cycles for birth control, The Guardian reports.

The hospital reportedly referred the app to Sweden’s Medical Products Agency.

In a statement, Natural Cycles, said it was “responding to each reported case”.

Last week, it was reported that health data from an Apple device was used as evidence in a murder trial in Germany.

German police believe activity data recorded by the Apple Health app, which suggested the suspect was climbing stairs at the time of the murder, could in fact have been him dragging his victim down a riverbank and then climbing back up.