Period tracking apps have become a familiar part of modern women's health, helping users record their periods, monitor symptoms and estimate when their next cycle may begin. But as these tools become increasingly intertwined with the booming femtech and wellness industries, experts are warning women to look beyond their slick interfaces and personalised promises.
A new analysis published in The BMJ on 30 September questions whether digital menstrual tracking and the increasingly popular practice of 'cycle syncing' are delivering the health and performance benefits often promoted online. Researchers say the evidence behind many cycle-syncing claims remains weak, while the sensitive nature of menstrual and reproductive data creates another reason for users to think carefully about how these apps are being used.
Cycle Syncing Claims Face a Reality Check
The BMJ analysis, led by Katharine M N Lee and colleagues, examined claims surrounding digital menstrual tracking and cycle syncing, which involves adapting exercise, nutrition or other lifestyle habits according to different stages of the menstrual cycle.
The concept has become a major wellness trend, particularly on social media, where women are encouraged to eat particular foods, change their workouts or adjust their routines depending on whether they are supposedly in a specific hormonal phase.
But researchers say decades of sports science research have found no generalisable effect of menstrual cycle phase on performance, injury risk or health. Where effects have been identified, they have generally been small and highly individualised.
Research into nutrition plans based around menstrual phases is also described as inconclusive, highly individualised or speculative. That means a routine promoted online as a scientifically tailored way to improve energy, fitness or wellbeing may not have the robust evidence users expect.
Your App May Not Know Your Cycle as Well as You Think
There is an important difference between tracking a period and accurately identifying what is happening hormonally inside the body.
Most digital menstrual tracking apps estimate cycle phases using information entered by the user, particularly the first day of menstrual bleeding. From there, an app can estimate ovulation, the fertile window and the timing of future periods.
The problem is that ovulation does not necessarily happen on the same day in every cycle. Hormone levels can fluctuate, and anovulatory cycles can occur, meaning simple calendar calculations cannot reliably establish someone's exact menstrual phase.
The BMJ researchers say measuring cycle phase accurately requires more than simply counting days. Hormonal testing, changes in luteinising hormone, basal body temperature and cervical mucus can provide additional information, but most standard tracking apps do not use enough of these measures to make highly precise predictions.
That does not mean period tracking is useless. Recording bleeding and symptoms can help women recognise patterns, prepare for their periods and communicate information to healthcare professionals. But users should be wary of treating an app's estimated hormonal phase as a medical fact.
Tracking Cannot Replace Medical Care
The researchers also warn that cycle tracking should not be confused with diagnosis.
Irregular or missing periods can have a range of causes, while painful periods can be linked to conditions including endometriosis and adenomyosis. Simply following an app's predictions cannot identify the underlying cause of these problems.
The BMJ analysis also highlights relative energy deficiency in sport, or REDs, which can affect menstrual cycles when there is severe and prolonged low energy availability. Adjusting workouts according to an app's predicted cycle would not address the underlying nutritional or recovery issues involved.
The Privacy Problem Is Harder To Ignore
There is another issue that women may not immediately consider when entering intimate information into an app: where that information goes.
Menstrual and fertility apps can collect sensitive details about periods, reproductive health and pregnancy. The US Federal Trade Commission has previously taken enforcement action involving fertility and period-tracking technology.
In 2021, the FTC settled with Flo Health after alleging that the company had shared users' sensitive health information with outside data analytics providers after promising that the information would remain private.
In 2023, the FTC also took action against Easy Healthcare, the company behind Premom, alleging that the ovulation-tracking app shared sensitive health information with third parties, including Google and marketing and analytics firms. The proposed order included restrictions on sharing health data for advertising and required user consent for certain other disclosures.
These cases do not mean every menstrual tracking app misuses personal information. But they show why privacy policies deserve the same scrutiny as an app's health claims.
What Women Should Look For
The BMJ researchers call for stronger protections for consumer and employee data, greater transparency about how information is collected and used, and stricter standards for apps recommended by healthcare professionals.
For users, that means checking what information an app collects, whether it can be shared with third parties, what privacy controls are available and whether data can be deleted.
Most importantly, period tracking and cycle syncing should not be treated as the same thing. Tracking symptoms and menstrual dates can be useful, but claims that an app can tell women exactly when to change their diet, exercise routine or performance strategy are much harder to support scientifically.
As cycle syncing continues to dominate wellness conversations online, the message from researchers is increasingly clear: women's health needs better evidence, better privacy protections and fewer promises that technology cannot reliably deliver.
