Most people use fertility apps to figure out when the best time is to conceive a baby, but it turns out there may be a completely different use for it - contraception.
The 'Natural Cycles' fertility monitor has unexpectedly proven to be as effective as the contraceptive pill at preventing unwanted pregnancies in a new study.
It predicts and identifies a woman's fertile window by tracking her period and predicting her ovulation day.
A new study analysing how effective the app was at preventing pregnancies among 4,000 Swedish women (aged 20-35) found the app to be almost as effective as the pill.
Despite the findings, sexual health experts said we need to be "cautious" of the method, particularly as - if not followed correctly - it could end in a high number of unwanted pregnancies.
Researchers said that among those who use the app correctly, five women out of every 1,000 will experience an accidental pregnancy.
For those who ignore the fertility warning or forget to input their temperature on certain days, seven women in every 100 will experience an accidental pregnancy during the first year.
To use the app correctly, women are required to record the temperature under their tongue and enter it into the app every morning.
It then determines how fertile you are on that day. If it's green, you're not fertile at this stage in time and the risk of pregnancy is very low.
If it's red, then you're extremely fertile, which means unprotected sex is a no-no. Unless, of course, you're looking to conceive.
The study found that 'Natural Cycles' has a Pearl Index of 0.5 and 7.0. Meanwhile the contraceptive pill has a Pearl Index of 0.3 and 9.0, the Telegraph reports.
This means that a very similar number of women are expected to get pregnant accidentally while using either 'Natural Cycles' or the pill.
With the contraceptive pill, minor side effects include mood swings, breast tenderness and headaches. In some cases, it can also increase the risk of blood clots and cervical cancer, says NHS Choices.
The app, however, does not interfere with a woman's body in any way and, as a result, there are no side effects.
The independent study was carried out at the Karolinska Institute, Sweden, and was published in the European Journal of Contraception and Reproductive Healthcare.
Study author and contraception expert Professor Kristina Gemzell said: "More and more women, especially in the age group of 20-30, tend to abstain from hormonal contraception and desire a ‘hormone-free’ alternative.
"It is important to increase choice among contraceptives for women and inform them about their pros and cons.
"This work is an important step towards understanding how new technologies can improve old methods."
Natika Halil, CEO for sexual health charity FPA, told The Huffington Post UK that interest in health apps is increasing and they potentially have "wide-reaching benefits in helping women avoid or plan pregnancies".
But we shouldn't necessarily rule out the pill.
"Natural Cycles is based on fertility awareness methods - also known as natural family planning - a method of contraception that has been around for a long time and suits many women," she explained.
"Current evidence suggests that fertility awareness methods are most effective when they are taught by a specialist and when more than one fertility indicator is used.
"Apps which only involve entering your body temperature and length of your cycle aren’t taking into account cervical secretions, which are another useful indicator of the fertile time.
"We already know that when used correctly, according to teaching and instructions, natural family planning can be up to 99% effective so we’re very interested to see the results of the Swedish study and would like to see further studies done."
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Halil added that women should be "cautious" of the results for the moment, particularly as other evidence suggests that typical use of fertility awareness methods - when they are not used according to instructions - can result in as many as 24 out of 100 women becoming pregnant in a year.
She continued: "Effectiveness depends on women being able to take their temperature and record it in the app every day, which won’t be suitable for some women in the same way the pill might not be suitable if you have trouble remembering to take it at the same time every day.
"And don’t forget that if you’re relying on an app you might not always have a working, charged mobile phone or tablet."
She noted that there are plenty of other contraceptive methods where effectiveness doesn't rely on a woman remembering to take or use them, such as the implant or an intrauterine device.
"It’s also important to remember non-hormonal methods aren’t for everyone – hormonal contraception gives a wide range of other benefits, including improving acne, controlling or stopping periods and improving PMS," she said.
While she said she would never recommend one method of contraception over another, she did say that women should opt for something based on what suits them at a particular time in their life.
"We want women to find a method that they are happy with that suits their lifestyle but it’s really important not to demonise the pill," she said.
"It may have a number of potential side-effects, and some of them are very serious, but they don’t affect all women and there are many benefits as well.
"A lot of women have really positive experiences on the pill and it can sometimes take a few tries to find a brand that works for you."
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