The app scans body temperature during the menstrual cycle to notify women when it is safe to have unprotected sex. Days when it's okay are marked as green on its calendar, but if it shows red couples are advised to use an extra form of protection. It's own research suggests Natural Cycles is more effective than the contraceptive pill when used correctly.

The 37 unwanted pregnancy cases stem from a study of over 600 women who sought abortions at Södersjukhuset hospital from September 2017 until the end of the year. "We have a duty to report all side effects, such as pregnancies, to the Medical Products Agency," midwife Carina Montin told Siren news agency. The findings could come as a blow to Natural Cycles' plans to pursue FDA approval.

In a statement, Natural Cycles said: "No contraception is 100 per cent and unwanted pregnancies is an unfortunate risk with any contraception.

"To have 37 unwanted pregnancies out of the 668 mentioned in this study at Södersjukhuset means that 5,5 per cent of women who stated they used Natural Cycles also had an unwanted pregnancy. This is in line with what we communicate as the risk of unwanted pregnancy with typical use, and which is comparable to other types of contraception."

In February Natural Cycles became the first app to be certified for contraceptive use, following approval by the German regulatory body Tuv Sud, after which its UK users spiked from 5,000 to 125,000. In November, the app, founded by husband and wife duo Dr. Raoul Scherwitzl and Dr. Elina Berglund, snagged $30 mIllion in funding.

Update: Natural Cycles has issued a new statement:

"Natural Cycles has a Pearl Index of 7, which means it is 93% effective at typical use, which we also communicate. Our studies have repeatedly shown that our app provides a high level of effectiveness similar to other methods.

"Natural Cycles has not yet received any information from SÖS, so we cannot comment on specifics. We are however in touch with the Medical Product Agency (MPA) and are responding to each individual reported case. 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. We have initiated an internal investigation with our clinical department in order to confirm this.

"As our user base increases, so will the amount of unintended pregnancies coming from Natural Cycles app users, which is an inevitable reality.

"We agree with what Carina Montin says "Perhaps young people should use another form of contraception". You have to be over 18 to use Natural Cycles. The average Natural Cycles user is 30 years and less than 1% are under 20.

"Today there is a big trend for women to move away from hormonal contraception, and Natural Cycles can provide a helpful option for these women. We therefore expect, in fact, overall to decrease the unwanted pregnancy rates because we're increasing contraceptive choice, and see that many of our users are women that have not used any type of contraception before.

"We'd like to reassure the medical community and the public that Natural Cycles is an effective, clinically proven, form of contraception, which hundreds of thousands of women worldwide trust as their birth control to prevent or plan a pregnancy.

"Our goal is to increase contraceptive choice so that all women find a suitable method of contraception."