Natural Cycles: A Contraceptive App?

Yesterday I watched Twitter blowing up in debate over an app called Natural Cycles. Marketed as an "effective method of contraception without side effects" and allowing you to "know exactly when you need to use protection", Natural Cycles have a huge online marketing presence with sponsored videos, blog posts and Instagram ads constantly popping up on my social media feeds. Watching this debate unfold made me feel like the only person in the blogosphere not being paid to use this thing, which is exactly why I wanted to write an unbiased stance on whether this app can be used as contraception, who it's suitable for, and how I'd advise starting out with this method of "contraception".

*I will add a disclaimer now that this post features periods, sex and fertility so if you are shy of these subjects please be aware that this post will not be for you.