Can I trust my period-tracking app? Here’s what it can tell you – and what to watch out for

As many as 50 million people now have a record of their menstrual cycle on their personal devices.

But how much should we rely on our period tracker for reliable and accurate information about menstruation and fertility?

Some apps are free, and some make you pay. All are part of a booming industry known as “femtech”. But evidence about the apps’ accuracy and quality suggest they don’t always help us better understand our bodies.

Here’s what period-tracking apps can – and can’t – 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’ll ovulate and your “fertile window”, usually about 3–7 days per cycle when conception is more likely.