In 2013, Clue founder Ida Tin had never used an app herself, but had an idea for creating a period tracker that would give people information about their cycle via their phones. Tin says phones felt like a natural home for it, with them already offering an intimate space that people interacted with on a daily basis, as well as an environment that was readily available.

At the time, there were already a few existing period-tracking apps, although Tin says they were mostly just calendars. “I had this idea of building a more mature product that could really help you understand the science behind it, that would be more educational and more visually appealing as well,” she says. “We had this idea of the cycle view, so you would instantly and intuitively know where you are on this monthly journey.”

Clue is now used by 10 million people a month, offers a Clue Plus subscription service for users that want deeper insights, and has published a comprehensive online encyclopedia that provides scientifically accurate information about contraception, menstruation, fertility and sex. Clue doesn’t take advertising, and has a no-sell data policy – something which more and more users are looking for, as stories emerge of medical apps sharing intimate information with advertisers. By all accounts, Clue is at the forefront of the rapidly growing femtech category, and if anyone knows what it takes to build a successful app like this, it’s Tin.