Your Period Tracker Is Probably Selling Your Data

When you open a period tracking app and log your cycle, you're sharing some of the most intimate data that exists: your reproductive health, your fertility windows, your physical symptoms, your mood. In a post-Roe v. Wade United States, in a world where data brokers operate largely without restriction, the question of where that data goes is not academic. It is a matter of personal safety.

We built Clio Daye because we couldn't find a period tracker that kept data strictly on-device and was open source enough to prove it. Before launching, we wanted to understand the landscape we were entering. So we scraped it.

The Market: 289 Apps, One Clear Winner

Using the App Store's search API across seven search terms and twelve countries, we identified 289 unique menstrual and cycle tracking apps currently available on iOS. The market is enormous, and almost entirely dominated by a handful of players.

Flo leads the market with 1.83 million App Store reviews, a proxy for tens of millions of active users. Clue follows at 391,000 reviews. After that, the numbers drop off sharply, with a long tail of hundreds of small apps competing for scraps of search visibility.

The top 10 apps collectively account for the vast majority of market share. Most users don't browse past the first few results. This matters because those top apps - the ones most people trust with their cycle data - have the most complex and commercially-motivated privacy policies.

The Illusion of "Free"

Nearly every app in this market (96.5%) is listed as free to download. But free doesn't mean costless. You pay with one of three currencies: a subscription fee, exposure to advertising, or your data.

Of the 18 top apps whose privacy policies we fully analyzed:

• 15 out of 18 share data with third parties
• 15 out of 18 contain advertising-related language
• 3 out of 18 have explicit data-selling language in their privacy policy
• All 18 require an account or cloud sync

The apps that claim "we do not sell your data" often do so in the specific legal sense required by CCPA, while still sharing your data with "partners," running behavioral advertising, and syncing everything to their servers.

Who Owns These Apps, and Who Do They Answer To?

One of the most important questions you can ask about a health app is: where is the company, and what laws govern them? We resolved developer origins for the top 50 apps by review count using company name suffixes, TLD analysis, WHOIS lookups, and bundle ID patterns.

The geographic picture is striking. Of the 289 apps we found, roughly 51% have origins we couldn't definitively resolve; many small developers use WHOIS privacy protection. Of those we could identify, the US dominates by app count, but UK-registered entities lead on user reach — driven almost entirely by Flo, which is incorporated in the UK but was founded in Belarus with its largest operations in Vilnius, Lithuania. The "UK" registration is largely a legal and financial structure, not a reflection of where the product is built or governed day-to-day.

* UK/HK combined outstrips the USA by 2.1x in total reviews, driven almost entirely by Flo (1.83M). Flo is UK-incorporated but was founded in Belarus and operates primarily from Vilnius — the UK registration reflects legal structure, not true origin.  ** Chart covers 285 of 289 apps; Japan (3 apps, 111 reviews) and South Korea (2 apps, 84 reviews) are included in the underlying data but omitted from chart groupings.

A Documented Pattern of Privacy Failures

These aren't hypothetical risks. Across the past six years, period and fertility tracking apps have been the subject of federal investigations, class action lawsuits, congressional inquiries, and independent research investigations — all finding the same thing: apps collecting your most intimate health data, and sharing it with people you never agreed to share it with.

Flo: the Market Leader with the Longest Paper Trail

Flo is the most-downloaded period tracker in the world, with 1.83 million App Store reviews. It was founded in Belarus in 2015, incorporated in the UK as a holding structure, and operates primarily from Vilnius, Lithuania. Its privacy history is also the most extensively documented:

Flo currently claims not to sell user data and has made policy changes since the FTC settlement. We note this not to single them out unfairly; by the standards of this market they are more privacy-conscious than many competitors. But it took two federal investigations and a $59.5 million settlement to get there.

Premom: Sharing Data With Third Parties While Claiming Not To

Glow: Fined by California's Attorney General

Stardust: Marketed as Encrypted, Found Sharing Data

After the Dobbs decision in 2022, Stardust surged to #1 on the US App Store by marketing itself as end-to-end encrypted and privacy-safe. A TechCrunch investigation found it was sharing users' phone numbers with third-party analytics firm Mixpanel, and that encryption keys generated on-device were being uploaded to Stardust's own servers — meaning Stardust could decrypt the data. The app subsequently removed all mentions of "end-to-end encryption" from its marketing.

Ovia: Selling Employers a Window Into Their Staff's Pregnancies

A Washington Post investigation found that Ovia sold a version of its fertility and pregnancy app directly to employers as a workplace "benefit." HR departments could access dashboards showing how many employees faced high-risk pregnancies, when new mothers planned to return to work, and what health questions staff were researching — all drawn from data users entered believing it was private.

Maya, MIA Fem, and Others: Sharing Intimate Data With Facebook Before You Even Opened the App

A 2019 Privacy International investigation found that several period apps were automatically transmitting intimate health data — menstrual cycle dates, mood, sexual activity, contraception choices — to Facebook the moment a user opened the app, regardless of whether the user had a Facebook account. Apps documented in the investigation included Maya (5M+ downloads), MIA Fem (1M+), and My Period Tracker. Four of the six exposed apps changed their practices after the investigation published.

The FTC Expanded Its Rules Because of This Industry

In July 2024, the FTC updated its Health Breach Notification Rule to explicitly cover consumer health apps, including period trackers. Apps are now required to notify users whenever their health data is shared without consent — not just when there's a hack. The rule was updated specifically because apps in this category had repeatedly shared sensitive health data without users' knowledge.

Natural Cycles: The Most Privacy-Conscious Cloud Option

If you must use a cloud-based period tracker, Natural Cycles (Sweden, FDA-cleared as a contraceptive app) has the strongest privacy posture in our analysis: governed by Swedish law and GDPR, no advertising detected, no third-party data sharing detected. However, it requires a mandatory account and your data is uploaded to their servers — that's simply how the product works. Once your data leaves your phone, you're trusting a company, a legal system, and a server you'll never see. Notably, in November 2024, Clue (Berlin-based) publicly stated it would refuse to comply with any US government subpoena for user data — a meaningful stance, but one that only matters if your data is stored somewhere that could receive a subpoena in the first place.

The Real Problem: Your Data on Their Server

Every app we analyzed that uploads your data faces the same irreducible risk: your data sits on a server somewhere you don't control. Servers get breached. Companies get acquired. Laws change. Government agencies issue warrants. In states where abortion is criminalized, period tracking data has been cited as a potential source of evidence. A privacy policy cannot protect you from any of that. Keeping your data on your own phone can.

Of the 289 apps we analyzed, we found only 3 that showed signals of true on-device operation with no account required, and none of them are open source, meaning you have to take their word for it.

Why Open Source Matters Here

Clio Daye is built on a simple principle: what we can't see, we can't promise. So we made everything visible. The source code is public. You can verify that there are no network calls. You can verify there are no third-party SDKs. You can verify that data is stored only in local UserDefaults and iOS Keychain. There is no server to breach because there is no server.

We're not the only privacy-focused app in this space, but we are the only one that's open source. That distinction matters because privacy is not a marketing claim; it's something that should be verifiable.

Clio Daye: Private by design, verifiable by code.

No account. No cloud. No third-party SDKs. 100% on-device. Open source.

What You Should Do Right Now

1. Check your current app's privacy policy. Look specifically for language about "sharing with partners," "advertising," and whether it requires an account.
2. Look up the developer's country of registration. EU-based companies are subject to GDPR; US companies are subject to inconsistent state-level protections; companies registered in tax havens may have minimal accountability.
3. Check if the app requires an account. If it does, your data lives on a server you don't control.
4. Consider whether you need a cloud-based app at all. For most users, cycle tracking is a local, personal record. There's no technical reason it needs to leave your phone.
5. If the app is open source, check the code. Privacy claims are cheapest to make in press releases and most expensive to fake in public repositories.