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Fertility-tracking apps can provide misleading information, and shouldn鈥檛 be relied upon to help conceive or as a contraceptive tool, according to an analysis of mobile phone apps available in the UK and Canada. The most reliable apps tend to be the free ones.

There are hundreds of apps that offer the ability to track periods in order to either achieve or avoid pregnancy. To find out how reliable they might be, Joyce Harper at University College London and her colleagues considered all 200 of the fertility-tracking apps offered on Apple鈥檚 mobile iOS app store.

First, they removed apps that were faulty, hadn鈥檛 been updated within three years and didn鈥檛 offer to predict a user鈥檚 鈥渇ertile window鈥�. This left 90 apps.

Just over half of the apps predicted this window based on when a user has their period. This 鈥渃alendar method鈥� is based on the understanding that, typically, ovulation happens on day 14 of the menstrual cycle, and fertility is highest in the days leading up to this.

But this method is flawed, says Harper. 鈥淵our menstrual cycle is not a textbook thing,鈥� she says. 鈥淣ot every woman has a 28-day cycle, we don鈥檛 all ovulate on day 14.鈥� In fact, Harper鈥檚 own recent research, based on the data entered into another fertility app, suggests that .

鈥淚n our study, women ovulated anywhere between day 10 and day 26,鈥� says Harper. Apps that rely only on dates are providing inaccurate information, and could result in some users missing their fertile window, for example, she warns.

The more robust apps took into account other markers of fertility based on user input, such as body temperature, which rises during ovulation, and information about the changing state of cervical mucus. Harper says she was surprised to find that, on the whole, the free apps used more of these biomarkers than paid-for apps.

Still, no prediction will be perfect, and only 57 per cent of apps provided any disclaimer to this effect. Just five of the apps advised against the use of the app for contraception. 鈥淭hey shouldn鈥檛 be relied on for contraception,鈥� says Rhonda Zwingerman at the University of Toronto in Canada, who wasn鈥檛 involved in the study.

That doesn鈥檛 mean that some apps won鈥檛 be useful. Apps that simply track period dates can help those who want to be reminded when their period is due, and might help someone notice when their聽periods appear to be unusually far apart, which could signal problems with ovulation.

But even the information on periods should be taken with a pinch of salt. As part of ongoing research, Harper entered the same dates into 10 different apps, which gave varying predictions for the date of the next period. 鈥淲omen shouldn鈥檛 worry if their period doesn鈥檛 come on the day the app says,鈥� she says.

Harper advises looking for an app that takes multiple factors into account to predict ovulation. But Zwingerman cautions that many can still carry inaccurate information.

Zwingerman鈥檚 own research found that around 22 per cent of such apps in other information they provided. 鈥淎lmost all of those claimed to be able to predict the gender of your child based on when you had intercourse in the cycle,鈥� she says. 鈥淛ust to be clear, that鈥檚 not how it works.鈥�