Caroline Davis had been using the Natural Cycles app as contraception for seven months when she discovered she was pregnant. "My period was a week late when I took the test," she says. "The 'You're pregnant' symbol came up immediately. I cried on the loo before phoning my boyfriend."

The 33-year-old nurse from south London wasn't ready for a baby; she had only been with her partner for a few months. After experiencing complications with the contraceptive pill, she had taken the risk of switching to the app - and now had to manage the consequences.

Natural Cycles is a 21st-century take on the age-old rhythm method of family planning, when women track their bodily changes and menstrual history to calculate their fertile window. Heavily marketed on social media (fertilitywith the algorithm to track her own cycle.

"The typical user uses it for a few years as a contraceptive before switching to get pregnant," says Dr Scherwitzl. "It's a product couples choose when they want to prevent pregnancies but are planning to have a child in a few years' time."

The average Natural Cycles' user is 30-plus and in a stable relationship - as per Sara Flyckt, 36, a former maître d at Somerset House's Spring restaurant.

A self-described "monster" when taking hormonal contraception, she was pleased to discover an alternative after giving birth to her son six years ago. Natural Cycles hadn't yet received a regulatory stamp of medical approval, but Flyckt didn't see it as a risk. "I never felt worried about using it at all," she says.