The fertility tracking app Natural Cycles is as effective at preventing pregnancy as the contraceptive pill

"Natural Cycles, the only app to be approved as a contraceptive, has proved 99 per cent effective in the largest study investigating it to date.
The startup had previously conducted a study of 4,000 women, which showed similar accuracy rates. It has now proven the efficacy of the app again after testing 22,785 women through a total of 224,563 menstrual cycles across a year, to calculate the app’s Pearl Index – the rate used to measure a contraceptive’s effectiveness. It found that if used perfectly – using protection such as condoms on red days – effectiveness is 99 per cent. Typical use, where people don’t use protection on the red days, leads to 93 per cent effectiveness, well above other natural family planning methods that rate at around 75 per cent and even the pill, which rates at 91 per cent."

Source:  Wired, 13th September 2017