Fertility apps which tell women when they are safe to have unprotected sex may be as good at preventing pregnancy as taking the pill, a new study suggests.

The first large, high-quality investigation of its kind found women who relied on an app which gives a green or red light depending on the day were at no worse risk of an unplanned pregnancy than those using some traditional forms of contraception.

More than 700 participants entered details of their period history into the Dot app, whose algorithm then predicted the pregnancy risk for each day of the menstrual cycle.

Over one year of use, the app had a failure rate of between one and five per cent, roughly equivalent to that of the pill and other methods.

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These comprised 24 pregnancies from women who used the app incorrectly, such as by having unprotected sex on a day of high fertility, but only one pregnancy form a women who used the app correctly.

The app had been enhanced with pop-up reminders, surveys, and instant messages for the purposes of the study.

"More and more women are using apps as a family planning method, so having an option backed by strong evidence of effectiveness is critical," said Dr Victoria Jennings, who led the trial at Georgetown University.

"Women must be able to base their app choice on solid evidence about how well the method works and what's involved in using it.

"That's why it was so important that an app like Dot undergo a rigorous effectiveness trial conducted according to established study guidelines used to study other methods."

The researchers found no association between pregnancy and socio demographic characteristics such as age, ethnicity, and relationship status.

"This is a particularly important finding because it suggests that Dot can be appropriate for a wide range of women," Dr Jennings added.

"Given the widespread use of mobile technology, a digital method like Dot has the potential to reach many women with an unmet need for family planning."

Claims made by contraceptive apps have been the subject of controversy in recent years.

Last year an advertisement for Natural Cycles was banned in the UK after figures showed it was less effective than advertised.

The smartphone app, which uses a thermometer to track the user's fertility, has been certified and marketed as a contraceptive since February 2017.

The new study is published in the European Journal of Contraception and Reproductive Healthcare.