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 (fertility trackers are the most frequently downloaded kind of health app in the Apple Store), it costs NZ$55 a year and comes paired with a thermometer, used to log your temperature before you get out of bed every morning.

iSTOCK One woman only used the app a few months before discovering she was pregnant: "I cried for two weeks."

Post-ovulation, progesterone warms the body by up to 0.45C, so an algorithm uses the data to calculate when users are likely to be fertile and gives them green and red days; the latter when they should abstain from intercourse or use additional protection, such as a condom. Natural Cycles says it only gives green days when it is 99.95 per cent certain. Initially, is shows around 40 per cent red days, climbing to 70 per cent after a few cycles.

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Natural Cycles The birth control app claims to be "as good as the pill" at preventing pregnancy.

But Davis, who says she followed the app's guidelines to a tee, isn't the only woman to have become pregnant when using Natural Cycles. In the last four months of 2017, the Stockholm South General Hospital in Sweden reported that it had helped 37 women terminate unwanted pregnancies conceived despite using the app.

Although it is still early days, more than half a million women worldwide have turned to Natural Cycles since the app launched in Sweden in 2014. A year ago, the technology was certified by European regulatory body TÜV SÜD for use as a contraceptive medical device; a standard widely accepted in the UK. And its popularity has grown - 127,000 downloads and counting - since a study of 22,785 women published in the journal Contraception in August found it was 99 per cent effective under "perfect" use and 93 per cent effective under "typical" use. The contraceptive pill, by comparison, is 99 per cent effective with perfect use but 91 per cent with typical use. The majority of users, around 80 per cent, use it for contraception rather than fertility tracking, according to Dr Raoul Scherwitzl, who co-founded Natural Cycles with his wife Dr Elina Berglund, 32, a Swedish particle physicist who came up with 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. "I used it the way I was meant to and stuck to the red and green days."

Switching to the app stabilised her moods and had a positive effect on her sex life. "Hormonal contraception affected how I felt about my partner and what I wanted," she says. "Natural Cycles improved that."

Flyckt used the app and thermometer until the summer of 2016. "When we decided we wanted to try for another child, I swapped it over from 'prevent' to 'plan'," she recalls.

"By then I knew my body and cycle quite well, so it only took a couple of months to become pregnant."

The Faculty of Sexual and Reproductive Healthcare, which sets standards in the UK, is yet to approve the app. Instead, it recommends that those interested in the natural method seek training through their doctor.

"The fertility awareness method is most effective when you use more than one fertility indicator, and when you're taught it by a specialised teacher," says Natika Halil, chief executive of the Family Planning Association. "Apps like Natural Cycles can help you understand your menstrual cycle better, and they can definitely be helpful if you're planning a pregnancy. But at the moment we don't recommend relying just on an app for contraception because there simply isn't enough independent research on their effectiveness yet."

Contraception that doesn't rely on perfect use, such as the implant and intrauterine devices, are more effective, she says, adding that it is concerning when technology companies feed into the narrative that "hormones are bad".

Caroline Davis had been using the combined contraceptive pill for five years when she was told it was no longer safe for her to do so. She was in her mid-twenties and had started getting migraines with aura, which are associated with a higher risk of stroke on the pill. Swapping to the progesterone-only pill made her feel terrible: "I bled every day for two months, so they upped my dose," she recalls. "The higher the dose, the worse I felt. I had symptoms of depression: I had no energy, wanted to stay in bed, and felt really emotional."

It was after hearing about Natural Cycles' certification that Davis decided to try it, and met her partner, who works in insurance, shortly after. By summer 2017, she had been using the app for around seven months and had reached the point when she had more green days than red ones. She inputted her temperature around 25 days a month and avoided unprotected sex when advised.

In July, she went to visit her new boyfriend in Amsterdam, where he was working. "He immediately said my breasts looked bigger," she recalls. "I ordered a beer and couldn't drink it; it didn't taste right." She was pregnant.

"I cried for two weeks. But both of us have always wanted a family and thought we would one day do it together."

Given her age and a diagnosis of endometriosis, Davis and her partner decided to continue with the pregnancy - he has now moved in with her and their baby is due on June 16.

Natural Cycles, which doesn't claim its method is foolproof, wasn't surprised by the reports of dozens of unwanted pregnancies in Stockholm. The Swedish capital, after all, is the app's hometown, and where it has the highest density of users.

Dr Scherwitzl says the numbers were within the expected range. "No contraception is 100 per cent effective, including Natural Cycles," he says. "As the product gains in popularity, it's not unexpected that a small fraction of users will experience unplanned pregnancies."

Sweden's Medical Product Agency agrees and has closed its investigation into the app's marketing. Natural Cycles hopes women will still consider the app and that the attention will encourage other contenders to enter the female technology market.

"I hope we're paving the way for future companies to move into this space," says Dr Scherwitzl. "Pharmaceutical businesses have stopped innovating in the field and we need more products available."

Davis, surprisingly, hasn't let her experience deter her from recommending Natural Cycles. "Despite what happened, both of us still think the app is a positive thing," she says. "We wouldn't say 'it's rubbish' or 'don't use it'. You can get pregnant on whatever form of contraception you use. Neither of us blame the app."

Some names have been changed.