One of the physicists who helped find the Higgs boson, Elina Berglund, has spent the past three years working on something completely different - a fertility app that tells women when they're fertile or not.

It's not the first fertility app out there, but Berglund's app works so well that it's been shown to help women avoid pregnancy with 99.5 percent reliability - an efficacy that puts it right up there with the pill and condoms.

Best of all, the app doesn't have any side effects, and just requires women to input their temperature daily to map their fertility throughout the month.

Back in 2012, Berglund was working at CERN on the Large Hadron Collider experiment to find the famous Higgs boson. But after the discovery of the particle, she felt it was time to work on something completely different.

"I wanted to give my body a break from the pill," she told Daniela Walker from Wired, "but I couldn't find any good forms of natural birth control, so I wrote an algorithm for myself."

The resulting app is called Natural Cycles, and so far, it's had pretty promising results.

Using a woman's natural fertility cycle to help her avoid getting pregnant isn't a new idea - it stems from something called the rhythm method, which is a form of contraceptive that claims to work just by having women avoid unprotected sex on fertile days each month.

In theory, that should work quite well. After all, there's only a roughly nine-day window during which a woman can get pregnant each month. But the rhythm method is pretty unreliable, seeing as all women have slightly different cycles, and in real life, it only has a success rate of around 75 percent.

But Berglund's algorithm is different - it uses the same advanced statistical methods she used at CERN, and is based on a woman's daily temperature rather than simply the day of her cycle.

That's because after ovulation, women see a spike in progesterone, which makes their bodies up to 0.45 degrees Celsius warmer.

So by entering your temperature in the app daily, and comparing the results with a broader dataset, the app lets you know when you can have unprotected sex (a green day) and when to use contraception, such as condoms (a red day).

There have been two trials so far, and the second one analysed data on more than 4,000 women aged 20 to 35 using the app.

Over the course of one year, there were 143 unplanned pregnancies in the cohort, 10 of which were conceived on green days, giving the app a 99.5 percent reliability rating. (The rest of the unplanned pregnancies were the result of women not using the app properly.)

To put that into perspective, condoms are 98 percent effective, and IUD devices are 99 percent reliable, as is the pill, when taken at the same time every day.

The latest trial was published in the European Journal of Contraceptive and Reproductive Healthcare.

Of course, Natural Cycles can't protect against STIs, so it isn't recommended for everyone. But for people who are having sex with a regular and trusted partner, the results so far suggest that it can work as well as more traditional types of birth control.

The app can also help women plan pregnancies, by taking the guesswork out of finding the best day to have sex.

But the real aim for Berglund now is to have the app classified as a contraceptive, not just a fertility monitor. "We are a natural alternative to the pill - with no side effects," she told Walker.

Not everyone is so convinced, though. In the latest trial, more than 1,000 women dropped out and stopped using the app over the course of the year, which shows that it can be hard to maintain. And women also have to be highly motivated and organised to record their temperatures at the same time every single day.

"It’s not a clinical trial but shows real-life performance," one of the researchers in the study, Kristina Gemzell Danielsson, from the Karolinska Intitutet in Sweden, told Wired. "True, motivation is key. For many women this is not the best method. However for motivated women it can be an alternative."

"Natural Cycles is not recommended to those who are very young or very keen to avoid a pregnancy, since there are other more effective methods," she added.

Those more effective methods are ones that don't require people to remember to take a pill, put on a condom, or record their temperature daily, such as intrauterine contraception or implants.

That's because human error can mess with things quite a lot. In fact, the UK's National Health Service (NHS) explained that when the app was used perfectly all the time, only five out of every 1,000 women would fall pregnant every year - a rate slightly better than the pill.

But for "typical use" - where the app isn't used entirely correctly every day - it's more likely that seven out of every 100 women would experience accidental pregnancies, which is around 93 percent efficiency.

They also reminded women that an app will never protect against diseases.

"However effective an application may be, it will not protect you against sexually transmitted infections, unlike the low-tech - but very reliable - condom," the NHS Choices blog explains.

Still, Berglund is working on improving the reliability constantly. The app now has 100,000 users paying £6.99 per month, and in June, the company received US$6 million in funding.

She's now hired another particle physicist from CERN to help analyse the data from the app and make it more reliable and personalised for each woman.

"It can be very scary, especially when it has to do with your body and your health," she told Wired. "We know we are dealing with women’s lives here and we take that very seriously."

But with women still waiting for the male contraceptive pill to be rolled out, and many experiencing negative side effects such as depression from other types of hormonal contraceptive, it's nice to know that some of the great minds in science are working on new options for us.