When Apple launched their health tracking app in 2014 and didn’t include any features that allowed women to monitor their reproductive health the company was widely ridiculed. As critics pointed out women have been tracking their periods for hundreds of years – in many ways they were at the forefront of the quantified self movement. Apple corrected their blunder but it drew attention to the nascent group of start-ups who were developing apps that claimed to track a woman’s fertility.

Today, the choice is staggering, from Kindara – a free app that allows you to track all manner of fertility markers, from your body temperature to the appearance of your cervical fluid – to Ovia, which includes in-app purchases, and allows you to monitor your moods and periods as well as other metrics.

While the marketing and details vary, the message is the same: use this app and you can pinpoint when you ovulate, and hence the days when you are most fertile. Packed with machine-learning algorithms, these apps promise to become tailored to your cycle, offering unprecedented insights into your fertility.

But while some are aimed at those trying to conceive, others include a setting for those who are not, allowing women to use the technology as a form of contraception.

Among the most heavily promoted is that of the Swedish company Natural Cycles, which offers an app that costs about £40 a year and comes with a thermometer. “It’s protection with more sexual freedom – minus the side effects,” says the website.

But while the technology might be new, the concept is not. Modern fertility-awareness-based contraceptive methods have their roots in the work of gynaecologists Hermann Knaus and Kyusaku Ogino, who in the 1930s independently explored the poorly understood concept that women have only windows of fertility during their menstrual cycle, revealing that ovulation occurs in the mid-point of the cycle. Their discovery led to the development of a range of contraceptive practices, from the high-risk approach of simply tracking menstrual cycles on a calendar aided by paper-based dials like the nattily named “Rythmeter”, to lower-risk methods involving daily temperature readings and checking the consistency of cervical mucus.

“Each day the woman feels the texture of her secretion between finger and thumb. For most of the cycle it is scanty and sticky, but just before ovulation (usually for about three days) it is more profuse and slippery,” says the 1984 Handbook of Contraceptive Practice, noting presciently that electronic devices to help calculate cycles were coming on to the market.

But it’s not that simple. For one thing, women’s cycles are unpredictable and can be affected by everything from stress to the common cold, and sperm can survive for days in the fallopian tubes.

Fertility tracking is a minority pursuit in the UK. According to the latest statistics, among women using NHS sexual and reproductive health services, just 1% used such “natural family planning” methods in 2015/16. In 2011/12 the figure was so low as to count as nonexistent.

It is, perhaps, not surprising. As David Grimes, a gynaecologist and expert in contraception at the University of North Carolina, notes, such methods rely on meticulous tracking by women, and, when fertile, she and her partner must either use condoms or other contraceptives or abstain from sex altogether. “It should be thought of as periodic abstinence, that is the better descriptor of it, rather than natural family planning,” he says. “For many couples, remaining sexually abstinent for more than half of the menstrual cycle is highly unnatural.”

The upshot, he notes, is that many of those who start using such methods soon quit. “They are very unpopular and very few people use them. And their main advocates are religious groups.” Not for nothing are fertility-based methods known as “Vatican roulette”.

But apps such as Natural Cycles have distanced themselves from traditional methods – and their sketchy reputation. Instead they are keen to establish the concept of contraception 2.0 by bringing a new level of accuracy to ovulation tracking.

Raoul Scherwitzl co-founded Natural Cycles with his wife, Elina Berglund Scherwitzl, after developing the technology for their own use. He says that previously women had to learn or take classes in fertility-awareness-based methods, “which are not cheap and not scalable for the general population”. Then they had to analyse the temperature themselves, and temperature charts, he says, are not as clear as the theory suggests. But the Natural Cycles algorithm “does all the work for the woman”.

Well, not quite all the work. Women still have to input their menstrual data and carefully record their temperature each day – a measurement that must be taken after a good night’s sleep and before getting up. Using the temperature data, together with optional additional information from hormone tests, the app tells a woman whether it is a “green” day or a “red” day – the former meaning she can have unprotected sex without the risk of a pregnancy, the latter indicating that she cannot. The more data is entered, the more days turn “green” as the app becomes better able to predict days of low fertility, with the algorithm taking into account a host of factors including irregularities in a woman’s cycle.

And as Raoul Scherwitzl points out, he and his wife know their onions when it comes to data – both are former physicists. Elina was one of the researchers that led Cern to discover the Higgs boson which got the Nobel prize. Raoul says: “What she did there was to basically analyse messy data and filter out a signal: look at what is the real signal, and what is just fluctuation.”

If you can detect the Higgs boson, the theory goes, predicting the release of a human egg should be child’s play.

But even data gathering is not without issues. Doing a workout the night before, being hungover, smoking, having sex that morning, feeling under the weather, travelling or being stressed all mean that a recording should be skipped, as body temperature might be affected, potentially leading the app to return more red days. It requires commitment and, if you want to have sex on a “red” day, other forms of contraception. With the burden, as with many contraceptive methods, falling on the woman, why not just use barrier methods and ditch the app?

“I think I speak for many men when I say that if you’re in a relationship you would like to avoid [condoms]. Just using condoms also leads to the fact that people skip protection and they skip it at the wrong moment,” says Scherwitzl. “With us it is a compromise. The woman does not expose herself to hormone contraception, and in exchange men have to pitch in a bit and use condoms on a couple of days a month.”

But not everyone is convinced that apps will bring greater sexual freedom. “It is hard to be free with your body when it requires a very regimented sexual lifestyle that most couples find unacceptable,” says Grimes.

Undeniably, there is a demand for hormone-free contraception that doesn’t involve an invasive device or necessarily rely on physical barriers. Condoms can interrupt spontaneity and, while many women use pills or long-acting reversible methods such as implants or intrauterine devices or systems without any bother, others have their reasons for not wanting to use them. Some women have had bad medical experiences: the pill has been linked to blood-clotting and depression, while some users of the contraceptive implant, IUD and IUS, experience irregular bleeding that can prove intolerable.

A quick glance at online forums tells all. “I’m tired of trying new [pills] and of pumping my body full of crap,” said one user of the Mumsnet website.

But just how effective are apps when it comes to keeping the stork at bay?

In an attempt to find out, companies have conducted a number of studies. In their latest, Natural Cycles looked at data from 22,785 paying users whose pregnancy status was determined either from them logging a positive pregnancy test or through follow-up messages from the team if users stopped using the app. For those who could not be contacted, the team estimated the likelihood of their being pregnant.

The results reveal that 8.3% of women using the app in a typical manner – a measure that includes slip-ups by couples – become pregnant in the first 13 menstrual cycles (the average number of cycles for a woman in a year).

By contrast comparable figures for typical use suggest that male condoms come in at about 18%, the implant at about 0.05%, traditional fertility-awareness methods at 24%, and the pill at about 9%, according to nationwide surveys in the US. These statistics scooped headlines. But experts still say the Natural Cycles figures should be used with caution.

“This is an utterly self-selecting group of women,” says Dr Sarah Hardman, deputy director of the clinical effectiveness unit at the Faculty of Sexual and Reproductive Healthcare, part of the Royal College of Obstetricians and Gynaecologists. “A lot of them will be the early adopters, the ones that actually were going out looking for something different, perhaps because they have had a problem with hormonal contraception – they are very motivated.”

What’s more, she adds, the study did not take into account whether the women had any underlying fertility issues, and relied on women self-reporting their data – including when they had sex and responding to questions of whether they had become pregnant.

“We don’t know what the quality of the data actually is, but it is probably not brilliant – and it is incomplete because they clearly haven’t reported some things like sex in cycles when they got pregnant,” she says, adding that in clinical research, by contrast, women are actively followed up for data. “They don’t tell us about whether people were using emergency contraception as well,” Hardman adds. “That could affect the overall pregnancy rate they were seeing.”

The upshot, she says, is that the figures must be taken in context. “This is not a bad study; this is an interesting study that is saying for this particular motivated group of women, of the women who actually ended up using it, their risk of pregnancy wasn’t too bad,” she said. Despite the drawbacks of the study, she adds, it could be difficult to recruit women for a randomised control trial, the gold standard for research.

Others were more scathing. “I wouldn’t call this a trial. It is an extension of marketing,” says David Grimes. James Trussell, an independent co-author of the study and an expert in contraception from the University of Princeton, rebutted the criticism: while the study was not a clinical trial, he said, its methods were on a par with similar research into other methods, including nationwide surveys.

“Sure, these people are motivated but anybody going into a clinical trial is going to be motivated as well,” he says. “[This research] is much more like a population-based survey, so the estimate of failure during typical use – these are the people who use the product,” he added, noting that all surveys rely on self-report.

And while the Natural Cycles study is funded by the company with the founders as co-authors, Trussell points out, much of the research into other methods has been funded by the pharmaceutical industry – a far from disinterested party.

Susan Walker, senior lecturer in sexual health at Anglia Ruskin University, who was not involved in the research, agrees that the study was well conducted. “I have to say in terms of looking at the efficacy of a contraceptive method which relies entirely on self-report I don’t think you could have designed a trial more robustly,” she says.

Still, some say that such studies are not enough. “Doing high-quality randomised trials of this method is not only ethical but urgently needed,” says Grimes, pointing to the dearth of such trials for fertility-awareness-based methods, as many “self-destruct” due to high drop-out rates.

Trussell is pragmatic. “Sure, you could do a clinical trial, but it costs a ton of money. There is never going to be a true clinical trial of the app.”

Scherwitzl adds that the company did plan to do a randomised control trial comparing the Natural Cycles app to the pill, but the trial was blocked by the Swedish authorities for legal reasons. “It’s a whole new concept to compare the impact of information with a pharmaceutical drug. We will give it another attempt,” he said.

The experts do appear to agree that, like all contraceptives, fertility awareness apps will not work for everyone – more than 4,000 women downloaded the Natural Cycles app but recorded little or no data, while of those included in the study, 54% stopped using the app within a year.

And, as with many traditional methods, slip-ups happen. “Over the past six months or so I have seen a small increase in women informing me they have used contraceptive apps, because they want a more natural method,” says Tracey Forsyth, nurse manager in contraception and sexual health at the British Pregnancy Advisory Service – the leading provider of abortion services in the UK. “The reason the method fails is that the client wasn’t able to take their temperature and took a risk and had sex on a “red day”.”

Scherwitzl admits that the app is not a contraceptive panacea, noting that, so far, users tend to be women in their late 20s, who are in a stable relationship and plan to have children in the not-too-distant future. But, he adds, for couples happy to gather data and use condoms on red days “it is a great option”.

Others agree. “There’s a small group of women who would find this helpful, but it is by no means a silver bullet,” said Walker: for some couples it could work extremely well, while young women, those with more chaotic lifestyles or those nearing the menopause who have irregular cycles might not find it so useful.

Hardman is more cautious. “If this is really what [women] want to use and they are absolutely motivated to use it properly – and they realise that it is only going to be as good as the effectiveness of the condom on the day that they are not supposed to be having unprotected sex – then it is a method of contraception that they could consider,” she said.

Bekki Burbidge, from the Family Planning Association, says that fertility tracking could also help women not on hormonal contraception to understand their body better. But, she adds, it is important women are aware of what is involved in using the app as a contraceptive method. “They need to know up front, before they spend money on the subscription, whether it is something that might work for them,” she says.

And for that, says Hardman, statistics are only part of the story. “It is a whole conversation with a woman, not just a number.”

…And, finally, why isn’t there a male pill?

Men can’t be trusted with contraception
The idea that men wouldn’t be willing to shoulder the responsibility and potential risks of contraception is widespread – yet consistently contradicted. According to Richard Anderson, professor of clinical reproductive science at the University of Edinburgh, the misplaced perception that women would not trust men to appropriately use a contraceptive has also been “a real bugbear”. His surveys of women at family planning clinics found huge enthusiasm for the burden of contraception to be shared between partners.

Big pharmaceutical companies have an innately male-chauvinist culture
The slow progress of male contraceptives has led to accusations of institutional sexism. “Most pharmaceutical industries are run by middle-aged men who aren’t terribly interested in this sort of thing,” agrees Anderson. “There is a perception, certainly in the developed world, that there already is an adequate provision of contraception and therefore a new method is going to lose a bit of the market for an existing method.”

Male contraceptives are too high an investment risk
Developing any new pharmaceutical product requires huge amounts of investment. However, the assumption that this is the sticking point for male contraceptives doesn’t correlate with the rapid development of new female treatments. Implants and hormonal coils for women have enjoyed huge recent success, despite neither being cheap to develop or market.

Male reproductive biology presents an intrinsically greater challenge
On average, men produce about 1,000 sperm per second for the entirety of their adult lives. Interfering with this is a tall order when compared with the monthly release of a single female egg. According to Anderson, non-hormonal male contraceptives being developed could rise to this challenge as they now represent a new approach, avoiding “the prejudices of how we regulate reproduction”.

Licensing a male contraceptive might be a difficult process
A male method presents a different balance of benefits and risks: longer reproductive lifespans could require men to use contraceptives for longer, and the health hazards of unplanned pregnancy do not directly apply. “It’s a complicated situation,” says Anderson, “in which – very unusually – one person is taking a drug, and another is getting the benefit.”

The range of available contraceptives is already sufficient
According to Anderson this perception is flawed: “If there are problems, you can run out of options pretty quickly.” Professor John Guillebaud, of University College London, agrees: “A lot of people don’t think this matters, but it does. A contraceptive that made a man temporarily infertile would be of enormous benefit to many couples, such as those where the woman cannot take the pill for medical reasons.” Julia Stanyard