A hospital in Stockholm has launched a formal complaint against a contraceptive app which they say is responsible for causing 37 unwanted pregnancies. Södersjukhuset hospital reported the app to Sweden’s Medical Product Agency, saying that 37 women have gone to the hospital for abortions since using the app.

Natural Cycles tracks a user’s menstrual cycle, as well as other factors like body temperature, to determine when a woman is fertile. The app was certified as a medical device for contraception in Europe in February 2017, owing to its specially developed algorithm that makes it much more precise than the average cycle-tracking app. It currently has 700,000 users worldwide.

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In an email to The Verge, a company spokesperson wrote:

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“No contraception is 100 per cent effective, and unwanted pregnancies is an unfortunate risk with any contraception. Natural Cycles has a Pearl Index of 7, which means it is 93 per cent 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.”

A seven per cent margin of error may seem high, especially when the goal is to prevent pregnancy, but Dr. Daniela Caprara, staff physician in the department of obstetrics and gynaecology at Humber River Hospital, says it’s actually quite good. The one caveat: our bodies are not machines.

“Theoretically, an app [like Natural Cycles] should be able to track your fertility accurately, but things can happen unexpectedly in terms of ovulation. For women who have irregular cycles or who don’t ovulate at the same time every month, apps like this won’t work. Our bodies do not do the same thing every month.”

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Overall, she says fertility awareness (or the rhythm method) is about 75 per cent effective, while the birth control pill, patch and ring are 93 to 95 per cent effective — when used properly; used improperly, these methods are only about 85 per cent reliable. Hormonal and copper IUDs are roughly 99 per cent effective.

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“The fact that this app says it’s 93 per cent effective is quite good, but that’s only assuming people are meeting the standard model for ovulating at the same time every month and they’re using it correctly,” she says.

Natural Cycles measures a user’s basal body temperature every morning to monitor her ovulation, making it more accurate than the average fertility tracking app, but Caprara says this isn’t the best way to measure ovulation, pointing instead to traditional ovulation kits as more precise.

“A blood test and ultrasound that you would do in a clinic are the best way to test for ovulation. If you’re doing some homework outside the clinic, an ovulation kit that monitors your levels through urine is more reliable than basal body temperature.”