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Some apps are free, and some make you pay. All are part of a聽聽known as 鈥渇emtech鈥�. But evidence about the apps鈥� accuracy and quality suggest they don鈥檛 always help us better understand our bodies.

Here鈥檚 what period-tracking apps can 鈥� and can鈥檛 鈥� tell you about your cycle.

How do period trackers work?

Period trackers ask users to enter a large amount of personal data, including period dates, sexual activity, mood, symptoms (such as bloating or cramps), and energy levels. Based on this self-reported data, the apps use algorithms to make predictions.

These include when your period will arrive, how long it will last, the day you鈥檒l ovulate and your 鈥渇ertile window鈥�, usually about 3鈥�7 days per cycle when conception is more likely.

Some apps rely entirely on the length of the menstrual cycle to make their predictions (known as calendar-based apps).

Others also use biometric data, such as daily body temperature, cervical mucous consistency or hormone levels in the urine. These bodily changes聽, and can therefore increase the accuracy of ovulation predictions.

But many apps don鈥檛 include this data in their algorithms,聽. And many apps aren鈥檛 transparent about how they formulate predictions, and聽聽to most users.

Challenging the 28-day cycle norm

The textbook menstrual cycle has been understood as聽, with ovulation occurring at day 14, or 14 days before the next period.

叠耻迟听聽since the 1960s has shown the length of menstrual cycles can vary between individuals and from cycle to cycle. Now, recently published data from period tracking apps reveals just how varied they can be.

Collaborations between researchers and app companies mean we have access to millions of cycles from hundreds of thousands of people.

These studies have shown聽聽of women have a 28-day cycle, and聽聽ovulate on day 14. There is also considerable variation month to month. For example,聽聽showed cycle length varied for more than half of women by five or more days.

These findings聽聽our definition of a 鈥渘ormal鈥� menstrual cycle. Despite this, many calendar-based apps still predict ovulation as 14 days before the estimated period start date, meaning they may be inaccurate for most people.

Should I trust app predictions to prevent pregnancy?

Some people use period app predictions for fertility planning, either to increase their chances of conception or as a contraceptive tool.

Although data examining apps鈥� ability to prevent pregnancy is limited, existing聽听丑补惫别听聽a fail rate of 7.2鈥�8.3%. This rate of unintended pregnancy is similar to that of聽聽with typical use.

However, studies often excluded participants if their聽聽somewhere between three and seven days. This means their effectiveness for preventing pregnancy is less certain for many people whose cycle varies.

The reliability of these findings also depends on how consistently users logged their data, which is hard聽. Many of these studies were also funded and authored by the companies that own the app studied, so results should be interpreted with caution.

Do period trackers help women learn about their bodies?

础蝉听聽has shown, apps often market themselves as empowering users, enhancing self-knowledge and control. Many claim to fill the gap in women鈥檚 health research.

But one聽聽study found people who use period-tracking apps are no more likely to have basic knowledge of their cycles than those who don鈥檛 use them. Another study found apps improve knowledge, but聽.

If the app鈥檚 prediction is wrong, users may聽聽rather than the app鈥檚 accuracy. Women have reported feeling stressed and anxious when the聽.

The quality of information on these apps can also be聽. Evidence-based information can help destigmatise and normalise common symptoms such as vaginal discharge. But when good information is available, it is often locked behind a pay wall or very generic.

Some apps may also endorse non-scientific beliefs, such as聽聽(planning workouts and diets based on menstrual cycle phases),聽聽(eating different combinations of seeds in an attempt to balance hormones) and聽.

So should I use a period tracker to monitor my cycle?

Period trackers are a聽, collecting personal information on a global scale. The companies behind these apps are聽聽by medical legislation, but instead by the rules of the app store that hosts them.

Provided you enter dates correctly, apps can give you an instant record of your menstrual cycles. This can be very helpful for a number of reasons, such as discussing your聽聽with a doctor, estimating your due date if pregnant or helping inform a diagnosis of menopause.

Predictions can also be useful as an idea of when your period may arrive. But be aware they could be wrong. And given the mounting evidence showing just how much menstrual cycle length can vary, don鈥檛 trust your app to tell you if your cycle is not normal.

While major irregularities can be cause for concern, irregular cycles are common and don鈥檛 always聽聽an issue. If you are concerned, it鈥檚 best to speak to your doctor.

This article was first published by and was authored by Dr Tessa Copp from the School of Public Health in the Faculty of Medicine and Health and Dr聽Emmalee Ford from the University of Newcastle.聽