Vicky and Jonathan were 23 when they got married and started trying for a baby, and it hadn't occurred to them that they might have any trouble conceiving. That was in 2007, but after a year, Vicky still wasn't pregnant. "It didn't sit easy, because we were both so young," she says. And after a GP referral and a number of tests, they were none the wiser: "They couldn't find anything wrong with either of us."

Fertility problems are estimated to affect one in seven couples in the UK – about 3.5 million people. About 84% of couples who have regular unprotected sex conceive within a year, but those who don't are diagnosed as infertile. And a quarter of them, like Vicky and Jonathan, are given a diagnosis of unexplained infertility – doctors simply cannot find a reason for it. That diagnosis was a mixed blessing for the couple. On one hand, knowing that there was nothing physically wrong gave them hope that they would one day be able to have children. But some days were darker, Vicky says: "On a bad day, you think 'if they can't find anything, no one can fix it for us either'."

It seems surprising that a process that appears so simple and natural should remain so mysterious in such a large proportion of cases. But new technologies, in the forms of apps and wearable sensors, are now helping couples to better understand the intricate physiological processes that influence their fertility, and so boost their chances of pregnancy. By gathering data from large numbers of women around the clock, these apps are also providing researchers with masses of information from which to look for broader clues and patterns about lifestyle and health factors that affect fertility more broadly, which could help other couples too – not just those using the apps. "We get amazing insights into fertility," says Paris Wallace, chief executive and founder of Ovuline, a company that has produced one such app, called Ovia Fertility.

For many women, the science of conception is already a refined one. Taking their basal body temperature measurements first thing in the morning or using over-the-counter ovulation kits are probably the best ways to determine when they are ovulating, and therefore the times when conception is most likely to happen.

Apps such as Ovuline's Ovia or Glow, another popular alternative, help refine this process by allowing women to input such daily information as temperature readings, when their periods occur, what their emotions and moods are, and to track sexual activity and even sexual positions. But Dr Allan Pacey, a fertility expert at the University of Sheffield, is not convinced that these apps are better than traditional methods. "Women have charted their menstrual cycles in a diary or put notes in a calendar for years," he says. "Apps that allow you to input that are just an electronic diary. If you want to use an iPhone rather than a pen and paper that's fine. The question is, how much will it help?"

Wallace says the apps teach users how to take control over their fertility. "For example, many people don't know when ovulation happens or that a couple can only conceive during six days of a cycle until they start using [it]." The manufacturers of the apps claim to use this information to give users tips on how to get pregnant faster. For instance, Glow will calculate the probability of a woman getting pregnant on any one day, and send reminders to her phone to tell her when she's most fertile.

Many of the users – 20% in the case of Ovuline – are infertile, and have turned to the app as an alternative to IVF and other assisted reproductive technologies, or an addition to them. Wallace says there is evidence to show it helps. "The average couple takes four to six months to conceive, but our users do it in 60 days, which is two to three times faster than the national average."

Of course, the same principles can also be used to avoid pregnancy. Apps such as Kindara and OvuView promote their use as a means of natural contraception as well as a way to help pregnancies happen.

One user, who wishes to remains anonymous, says: "I think these apps are really good for people who can't have sex regularly for whatever reason, or who don't really understand their cycle due to not understanding the science or not having regular periods. But if you are capable of having sex two to three times a week, then they're not much use except for peace of mind – seeing you've ovulated [because of] a temperature change, for example."

Where the technology becomes arguably more interesting is that these apps are increasingly becoming compatible with others such as food diaries and activity trackers, allowing data to be used to look for other possible factors in conception. These aren't things that doctors can glean from talking to individual patients. But with hundreds of thousands of people inputting data, it's a different matter.

"At Ovuline, we're undergoing the largest research study in reproductive health in the world," says Wallace. "Our users have submitted over 100m data points about their health and lifestyles, and we're getting 24,000 new data points every hour." Working with scientists at Harvard University, the company is undertaking two research projects – one on the signs of early pregnancy, for instance how your emotions can indicate whether you are pregnant, and the other on a method of "scoring" a person's fertility.

There is clearly a huge market for this kind of technology. Ovuline claims to have hundreds of thousands of users (although it will not reveal exact figures) and says the number is growing by 20% each month. But although these apps are supposed to make fertility cycles easier to track and analyse, for some people, manually inputting the information doesn't work. Vicky, who is herself a midwife, gave up taking her basal body temperature to track ovulation after two months. The temperature has to be taken at the same time every day, and with her shift work it was too difficult. And taking daily readings was adding to the pressure she already felt. The infertility issue "was ruining my life enough without trying to take my temperature every day and do all those things," she says.

After they had been trying for a year and were therefore told they were infertile, Vicky and Jonathan decided to try IVF, but swore that they would only try as many times as they could on the NHS, to avoid getting into debt. With the news that the second round of IVF had not worked, the couple went back to "normal life".

By 2012, they were just beginning to get their heads around the idea that they might never become parents. "We were starting to accept it as much as you can. We started to look at expensive holidays and treating ourselves because we had put our life on hold to try for a baby," Vicky says.

It was then that a colleague told her about a new gadget called DuoFertility, which they thought looked interesting. Her colleague was having similar fertility issues, and "we both agreed we would do it together because we had nothing to lose," Vicky says.

DuoFertility was launched in 2009 by Cambridge Temperature Concepts, a Cambridge University spin-off, and has been steadily growing in popularity over the last couple of years as a result of the boom in both "big data" analytics and advances in sensor technology.

Women who are looking to conceive wear a sensor on their skin under the arm. It monitors very small changes in their body temperature and can more accurately detect the small increase in temperature associated with ovulation.

Rather than having to manually take a reading at the same time every morning, the sensor constantly takes them: about 20,000 times a day. It then wirelessly transmits the information back to a small receiver, which can be plugged into a computer for analysis. By picking up small changes in body temperature, the device provides a much more accurate picture of when the woman is ovulating. "Simply having this information on precisely when to try is shown to make a dramatic impact on a couple's chances," says Claire Hooper, DuoFertility's chief executive. As with other apps, users also manually input data such as sleep, physical activity levels and their sense of wellbeing. "We are able to establish patterns and anomalies that may be missed on a single visit to the clinic," she says, because the device monitors people as they go about their daily lives.

The company also prides itself on the fact that it provides a personal support and analysis service for each couple. DuoFertility staff analyse the data and send emails providing help or asking for more information to provide context. For instance, Vicky would tell them what shifts she was working because warm temperatures on the ward could affect her body temperature. The team also gets in touch if there seem to be any problems that might warrant a visit to a GP – such as signs of polycystic ovaries.

Vicky and Jonathan started using the device in February 2012 and found out Vicky was pregnant by the end of April – three cycles later. After five years of trying and two courses of IVF, it had worked after just a few months, and their son is now one year old.

Vicky speculates that success might have just been down to the fact they were more relaxed and knew that they did not have much to lose. The most popular DuoFertility package costs £495, which includes the sensor itself, the wireless monitor to which the data is transferred, and the one-to-one support over the phone and by email.

Hooper says people use the service for a range of reasons – some are "simply very busy and need to schedule their time together – we can give up to six days' advance warning of when to try, which is exactly what they need to align their diaries!" she says.

Others are often considering IVF because of unexplained infertility but would like to be sure they have maximised their chances of a natural conception first. Some are between cycles of IVF, or, like Vicky and Jonathan, have tried it without success. In 2012 DuoFertility was approved as a medical device by the US authorities, and peer-reviewed research conducted by the company has found significant increases in pregnancy rates of people using the device. "In the UK, one in eight with unexplained infertility who have been trying to conceive naturally for more than two years, but less than three, will get pregnant within a year," says Oriane Chausiaux, DuoFertility's co-founder and chief scientific officer. That's a 12.5% pregnancy rate. "For a similar group of couples, we have 39% pregnancy rate with DuoFertility over 12 months." The company claims that for many couples the service is a viable alternative to IVF.

It's important to note that this is just related to people with unexplained fertility or mild problems – the device cannot help people who have serious conditions that are preventing them from conceiving naturally. And the device doesn't work in all cases of those with unexplained infertility . The colleague who first told Vicky about DuoFertility had no success.

Meanwhile Glow has come up with a way to make the app into a kind of insurance should couples fail to conceive. Through the app's Glow First programme, users can choose to make a monthly £30 contribution until they get pregnant. Those who don't conceive after 10 months are entitled to money from the kitty to help pay for IVF. "It's crowdfunding for babies," says Glow co-founder Mike Huang.

Hooper says in the future, DuoFertility would like to be able to use the sensor to understand other conditions. "We're collecting and cross-referencing huge quantities of patient data. When analysed together, all this data is especially relevant to other aspects of healthcare such as pain management, sleep problems, obesity and other conditions. It's a prospect we are actively exploring."

For Vicky, being able to contribute to something bigger is also an exciting prospect. "It's so un-invasive, no one needs to know you are doing it. Yet all that data that you are giving them, hopefully will find things out for the future and help so many other people."