Are women let down by period trackers?

When journalist Orla Barry received a notification from her iPhone informing her that her period was due "any day in the next three weeks", she shared it on social media with wry amusement.

It wasn't the first time she'd received such an unspecific notification from the app, and it prompted others to share their stories.

"I got one which said my period was 56 days late," wrote one.

"My notification said 'the next nine days'," said another.

One man said his smartwatch had a menstruation tracker activated by default when he got it, and it kept telling him his period was "due" - despite him never having had one.

These apps do face a big challenge - periods are not always renowned for their punctuality. But are they up to the job?

At their most simple, women input the dates when their periods begin and end, and an app calculates when their next is due to arrive based on this information. It can also use this data to estimate when they might ovulate: this is also when they are most likely to conceive.

Some offer to track additional data including basal body temperature, sleep patterns, menstrual pain and sexual activity, which can provide further clues - although there have been concerns around what else this data can be used for by the developers of the app.

However, women's cycles can change from month to month based on a large number of factors including stress, age, and hormone fluctuations.

It is perhaps not all that surprising that a scientific study of nearly 1,000 women carried out in 2018 found that the apps they were using were only correctly identifying when they ovulated 21% of the time.

But period trackers remain very popular. They are used for a number of reasons, including:

Laura Symul, a postdoctoral fellow at Stanford University, studies period trackers and the data they collect. She says that these multiple reasons for using the same product form part of the problem, especially when a woman is likely to have different reasons for using one at different times in her life.

"There are very few apps that have really been designed for the full experience of what it is to have a menstrual cycle," she says.

"At first, someone may just like knowing when their period comes, and then they want to get pregnant and suddenly the tracker they've been happy with for years is not working for them any more.

"Then there is the same thing when they become pregnant, their new app is not useful and then they have to go to a pregnancy tracker."

Laura thinks what is needed is one app which can guide women right through from puberty to menopause.

There's certainly plenty of caution around how well existing apps are performing.

In a blog published last year, Dr Jessica Chan, a fertility and reproductive medicine expert at Cedars-Sinai in the US, said that period trackers on their own are not reliable enough for those who are trying to start or grow a family.

"Unless someone is checking ovulation through physical means, like by using an ovulation prediction kit, the cycle app is just providing an estimate of when their fertile days are," she said.

Neither do these apps get a ringing endorsement as a form of contraception.

Natural Cycles is the only such app to be approved by the US Federal Drugs Administration (FDA) for this purpose.

Women using Natural Cycles have to input their temperature every day and the firm claims that, with "typical use", 93 women out of 100 who use it will not fall pregnant, giving it the same level of reliability as the pill.

Rebecca Woodhouse, who became pregnant in 2018 while using it, told the BBC she was led to believe it was more effective than that.

"I have days where I'm like 'oh, that stupid app'," she said at the time.

That said, Natural Cycles has more than 500,000 downloads on the Google Play store, and the vast majority of its 14,336 reviews have awarded it five stars.

For women who are perimenopausal and trying to make sense of their changing cycles and symptoms, there is also a level of dissatisfaction.

Suw Charman-Anderson uses the popular tracker app Clue and says: "There's nothing that allows me to easily track just how variable my cycle is, other than eyeballing a chart that's buried several taps away."

She adds that it is also impossible to track other symptoms such as hot flushes, and says she has now set up her own spreadsheet to do the job instead.

"The app is all geared towards women trying to get pregnant - I don't really care when my fertile period is, I want to know how bonkers my cycle is," she said.

Clue offers a lengthy explanation about its limits as a fertility aid, but it is much harder to find content on its website about the menopause.

Clue says it does provide information about menopause and its symptoms to users aged 45 and over, but but admits that it is not "specifically tailored" towards that market.

The app has 13 million users worldwide and more than one million reviews on the Apple app stores, many of which have left it five-star ratings.

It says it uses an algorithm based on the previous six months' worth of data shared by users in its calculations.

"Clue is backed by science, but as with any sector there are inferior products out on the market, so it's sadly perhaps not a surprise that some people feel disappointed with their experiences," a spokeswoman said.

Of course period tracking itself is not new - generations of women have charted their menstrual cycles using old-fashioned pen and paper.

There are many apps on the market which are not really offering an improvement on that, says Carolina Milanesi, tech analyst at Creative Strategies.

She says some of the apps she has seen her daughter use lack "intelligence" and aren't doing much beyond marking time between cycles.

"You could put a reminder in the calendar based on when you had your period the month before and achieve the same," she said.