Bethanie Alys may have been only 20 years old, but she harboured ambitions of becoming a teacher before settling down to start a family of her own.

Fast forward a year, however, and she is mother to a four-month-old. Her career aspirations are in tatters and her sentences stilted through sleep deprivation.

The cause of her derailment? A ‘fertility app’ she installed on her smartphone to avoid getting pregnant during a fledgling relationship. Needless to say, it didn’t exactly do its job.

Bethanie Alys, 21, from Wolverhampton, unexpectedly became pregnant after using a 'fertility app' that is supposed to detect when it's 'safe' to have unprotected sex

Fertility apps - which track a woman’s bodily changes throughout her menstrual cycle to detect when she can conceive and when it’s ‘safe’ to have unprotected sex - might sound far-fetched, but more women than ever are relying on them to prevent pregnancy.

There are more than 250 versions available and astonishingly, a clinical trial has found one app to be almost as effective as the contraceptive pill.

The trial - published in the European Journal of Contraception and Reproductive Health Care - discovered that just five women in every 1,000 experience an accidental pregnancy within a year of using the Natural Cycles app, compared to three in every 1,000 who will get pregnant in 12 months when using the Pill.

With no hormones to ingest and minimal expense, it is easy to see the appeal of an app. But experts aren’t convinced. ‘While they may be helpful to predict peak fertility, they are by no means reliable enough to use as a failsafe method,’ says Dr Shazia Malik, consultant obstetrician at The Portland Hospital for Women and Children in London.

She is now mother to a four-month-old boy, called Lake. But she says she felt let down when she found out she was pregnant

Bethanie, 21, delights in baby son Lake but his conception was certainly a shock. ‘I am besotted with Lake, but when I found out I was pregnant I felt let down,’ she explains. ‘I never expected to become a mother this young.’

Bethanie, from Wolverhampton in the West Midlands, opted for a fertility app because the contraceptive Pill - which contains the hormone oestrogen to prevent ovulation - had left her moody and prone to weight gain.

The premise of a fertility app is straightforward. By pinpointing the date of ovulation - when an egg is released from the ovary - it identifies the timeframe of around six days every month when sperm can fertilise the egg.

When she starts using an app, a woman enters the date her period begins into a calendar. Ovulation typically occurs 12-14 days afterwards and the idea is that after a few months of monitoring a pattern will emerge.

Bethanie, pictured with partner James, was working as a languages teacher in Spain when she started using the app. This was as the contraceptive Pill had left her moody and prone to weight gain

More advanced apps also require users to enter their daily basal body temperature (BBT) - your temperature immediately on waking. This is the body’s lowest temperature, and an increase of just 0.2 degrees Fahrenheit can indicate ovulation.

As Dr Malik explains: ‘Once the egg is released, the cells in the remnants of that follicle in your ovary release a hormone called progesterone that raises body temperature the day after ovulation.’ Once enough data is collected, the app can predict possible fertile days.

Bethanie - at the time a languages teacher in Spain - started using her app in September 2014, five months after she and James, a 21-year-old university student, had become a couple but before they’d had sex. She downloaded a version called Period Diary, for 66p and bought a thermometer to test her BBT.

Her app marked the predicted date of ovulation on a calendar with a star, and the four days on either side of that date were coloured green, to signal she could get pregnant. ‘Over the course of three months the app moved the date of my predicted ovulation forward by three days as it took my BBT readings into account,’ says Bethanie. ‘Perhaps it was naïve, but I trusted the system.’

Lucy Phenix, 31, a bank cashier from Willesdon, Cheshire, is six months pregnant, having also conceived while using an app as contraception

However, last April, Bethanie experienced stomach cramps. She visited her GP who advised a pregnancy test. ‘When it proved positive I was shellshocked,’ she recalls.

It is entirely possible for an app to get ovulation dates wrong. Dr Malik explains: ‘Even women who have a predictable menstrual cycle only ovulate on the same day 80 per cent of the time and even if measuring their BBT some women don’t have that rise in temperature associated with ovulation.’

Bethanie broke the news to her mum Lisa, 45, a health care assistant who divorced her father when Bethanie was two. ‘She worried I was too young and hadn’t been with James for long enough,’ says Bethanie. ‘James also assured me we would manage.’

In October Bethanie returned to Wolverhampton. She took a job as a retail assistant and she and James, who is joining the Army after he graduates, rented a one-bedroom flat. Lake was born in January.

Lucy, who is already mother to Preston, three, and Prudence, 22 months, says she and husband Richard had only planned to have two children. She chose a free app that catalogued her cycle by the dates of her period

Despite the hardship of her surprise pregnancy, her relationship with James has strengthened, and the pair hope to marry next year. Surprisingly, she wouldn’t rule out using a natural method of contraception in future but says: ‘Next time I won’t trust an app as contraception.’

Lucy Phenix, 31, is six months pregnant, having also conceived while using an app as contraception. The mother to Preston, three, and Prudence, 22 months, is overwhelmed by the prospect of a third baby. ‘We only planned two children and I am still in shock,’ says Lucy, a bank cashier married to Richard, 35, manager of a plumbing company.

She started using an app called Fertility Friend last summer. Although it can be upgraded to a version that tracks BBT for £30 a year, Lucy chose the free basic model that catalogued her cycle by the dates of her period. Given she was desperate to avoid pregnancy it was a decision many would believe foolhardy, but Lucy explains: ‘I was diagnosed with polycystic ovaries in my 20s and conceived my children only after taking hormone-regulating drugs, so I thought the chances of falling pregnant were minimal.’

Sara Flyckt, 34, a maître d’ from London, has been using the NaturalCycles app for 18 months following the birth of her son Mio, now four

Yet last December, six months after installing the app, a pregnancy test proved positive. ‘I was in a state of disbelief,’ says Lucy. ‘Richard was as shocked as I was. We’d only had sex twice that month. I must have conceived a week before the app said I was fertile.’

The couple are hurriedly building an bedroom extension onto their three-bedroom home in Willesdon, Cheshire, and have bought a seven-seater car.

‘The idea of a third baby is daunting, and looking after two small children while pregnant is exhausting,’ says Lucy. ‘But I would never wish this baby away now.’ Richard will be having a vasectomy afterwards and she says: ‘I would never rely on an app as a form of contraception again.’

The makers of NaturalCycles are quick to distance its app from rivals, claiming the software is more advanced. Created by physicist Dr Elina Berglund and launched in 2014, it costs £6.99 a month or £49.99 for a year but is the only one to have been validated in a clinical trial.

Sara says she was sceptical at first but did not want to go back on the contraceptive Pill. She says it has been empowering to understand the changes in her body every month

Sara Flyckt, 34, from London, has been using the NaturalCycles app for 18 months following the birth of her son Mio, now four. ‘I was sceptical but I didn’t want to go back on the contraceptive Pill,’ says Sara, who works as a maître d’ and has been in a relationship with Carlo, 30, a chef, for seven years.

She uses a thermometer bought from NaturalCycles to take her BBT. Users of the app also enter the results of an ovulation test which detect levels of a luteinising hormone present in urine prior to ovulation. It marks the days in which users may be fertile in red.

‘I found it empowering to understand the changes in my body every month,’ says Sara.

Be that as it may, there will doubtless be those who think that when it comes to preventing pregnancy, a smartphone app is still not quite smart enough.