An advert for an app billed as a natural alternative to contraception has been banned after it was deemed to be misleading.
Natural Cycles, founded in Sweden, says it offers a clinically tested app that offers "digital birth control".
The UK's Advertising Standards Authority (ASA) has ruled that claims made in a Facebook advert about the effectiveness of the app were exaggerated.
The ad, which ran for four weeks in mid-2017, said: "Natural Cycles is a highly accurate, certified, contraceptive app that adapts to every woman's unique menstrual cycle.
"Sign up to get to know your body and prevent pregnancies naturally."
An accompanying video claimed the app, which charges customers £5.99 a month or £39.99 annually, said the service "offers a new, clinically tested alternative to birth control methods".
Natural Cycles has previously defended its technology after it was linked to 37 unwanted pregnancies, according to a report in Sweden.
The smartphone app uses a thermometer to track the user's fertility, and has been certified and marketed as a contraceptive since February 2017.
The Swedish company says the certification was based on clinical trials involving almost 23,000 women over two years
The app's algorithm takes into account factors such as ovulation day, cycle length and the average temperatures of different phases throughout the menstrual cycle.
It then assigns a number of green days to notify the user of non-fertile days, and red days – when the user is predicted to be fertile.
Based on its trials, Natural Cycles said the app had a typical-use failure rate of 6.8 – meaning 68 women out of 1,000 would fall pregnant during a year of use if the app failed, or if they had unprotected sex or did not use contraception on a red day.
Ten of those pregnancies (1%) would occur even when there was "perfect use" of the app, the company said – so either "specifically due to a falsely attributed green day" or when contraceptive methods failed during intercourse on a red day.
The trials led the firm to bill the app as having a 93% "typical-use" efficacy rate and a 99% "perfect-use" rate.
The ASA, which received five complaints regarding the app, said in a ruling: "In the context of the ad, the claim 'highly accurate contraceptive app' would be understood by consumers to mean that the app had a high degree of accuracy and was therefore very reliable in being able to prevent unwanted pregnancies.
"We further considered that the claim 'clinically tested alternative to birth control methods', presented alongside the 'highly accurate' claim would be understood to mean that the app was a reliable method of contraception which could be used in place of other established birth control methods, including those that were highly reliable in preventing unwanted pregnancies."
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The watchdog added that the "typical-use" effectiveness of the app was comparable to other contraceptive methods such as condoms but was "significantly lower than the most reliable methods", including the intrauterine device (IUD).
It further told Natural Cycles Nordic AB Sweden that they must take care "not to exaggerate the efficacy of the app in preventing pregnancies".