An analysis of fertility-tracking apps available in the UK and Canada found that many use flawed methods to predict periods, and they should not be relied on to help conceive or as a contraceptive tool
Source:
https://www.newscientist.com/article/2254856-most-fertility-tracking-apps-are-unreliable-but-free-apps-work-best/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home