Like most people, the first thing 32-year-old Jo Hellyer reaches for when she wakes up is her smartphone. Yet instead of checking her messages or emails, she opens a fertility awareness app and takes her Basal Body Temperature (BBT) to find out if she’s at risk of falling pregnant in the next 24 hours. Welcome to the world of “digital contraception”, a controversial new form of birth control.

Menstrual trackers are nothing new. In fact, many health professionals encourage their patients to monitor their monthly cycles this way – especially if they’re hoping to conceive. What is different here is that Hellyer is one of a growing number of women relying solely on algorithms dreamed up by developers to prevent unwanted pregnancy. Scroll through the smartphones of half a million women worldwide and you’ll find they have one download in common: Natural Cycles, the only fertility awareness app to be certified as a contraceptive in Europe and now the US.