Every "best private period tracker" list ranks apps on features, design, and price. Those matter, but they are not what makes an app private. Only one thing does: where your data lives. If your cycle history sits on a company server, it can be breached, sold, or subpoenaed, no matter how good the privacy policy sounds. If it never leaves your phone, there is nothing for anyone to take.
This is the axis we compare on. Everything else is secondary.
Why 2026 made this urgent
Two things changed the stakes. In the United States, reproductive health data became something people worried could be requested in court. And in July 2026, a Mozilla Foundation review tested six popular period trackers by watching what they actually sent over the network, not what their marketing claimed. The results were not reassuring.
The gap between "we care about privacy" and "your data never leaves your device" turned out to be enormous.
The 2026 Mozilla scoreboard
Mozilla scored six apps out of ten. We are reporting their findings, not our own tests.
- Euki: 10 out of 10. On-device, open-source, run by a non-profit. Mozilla called it "squeaky clean" and noted there is "almost nothing to leak" because the data stays on your phone.
- Clue: 8 out of 10. A clinically minded tracker based in Germany, operating under GDPR, but still cloud-based.
- Flo: 7 out of 10. The largest app, described by Mozilla as "a data-hungry tracker with an AI symptom chatbot." Flo previously settled with the US Federal Trade Commission over sharing sensitive data, and added an "anonymous mode" after 2022. The data still lives on Flo's servers.
- Period Calendar: 6 out of 10.
- Spot On (Planned Parenthood): 5 out of 10.
- Stardust: 2 out of 10, the worst score. This is the one worth reading carefully.
The Stardust lesson: marketing is not architecture
Stardust marketed itself as an encrypted, privacy-safe app. Mozilla's researcher found that its encryption was standard, not end-to-end, and that the app shared users' birthdates, birth control types, reproductive goals, and symptoms with an analytics company called RudderStack, tied to a unique identifier. Stardust said RudderStack is contractually barred from selling the data. That may be true. It does not change the fact that the data left the phone and now sits with a third party that can, like any US company, receive a legal demand for it.
This is the whole point. A promise you have to trust is weaker than an architecture that makes the promise impossible to break. Stardust is not unusually careless. It is a normal cloud app whose marketing outran its architecture.
Two categories, not ten apps
Once you sort by where data lives, the field collapses into two groups.
Cloud-based trackers store your history on their servers. Flo, Clue, Stardust, and most of the market live here. Some are careful, some are not, but all of them hold a copy of your reproductive history that exists outside your control. The privacy question becomes "do you trust this company, its vendors, and its future owners, forever?"
On-device trackers keep everything on your phone. Euki is the reference point, and it is genuinely excellent for privacy. Apple's built-in Cycle Tracking is also on-device and encrypted, though it is Apple-only and basic. The trade-off in this group has traditionally been fewer features and smaller teams.
How to judge any period tracker in thirty seconds
- Does it require an account or email? An account means a server-side identity attached to your data.
- Does it sync to the cloud? If yes, a copy exists off your phone.
- Can the privacy claim be verified? On-device claims can be checked with a network proxy that shows whether the app makes outbound requests carrying health data.
- Does it work fully offline? Real on-device apps do.
- How is it funded? Ad-supported and data-broker models have a reason to want your data.
If an app fails the first two, no policy language fixes it.
Where mylune fits
We are building mylune in the on-device category, the same structural approach that earned Euki a perfect score, and we are trying to close the trade-off that has held that category back.
mylune keeps every piece of cycle data on your device. There is no account, no cloud, and no server that holds health data, so there is nothing to subpoena. The privacy claim is meant to be verified, not trusted: mylune makes no outbound network request that carries health data, which anyone can confirm with a network proxy. It is free forever, funded by partnerships and grants rather than data, and ships in 18 languages with a Doctor Visit PDF export, app lock, Stealth Mode, and a Legal Protection Mode for US states where reproductive data carries legal risk.
To be straight with you: mylune is launching in early access in the United States, so it was not part of Mozilla's 2026 test. We would welcome that scrutiny, and we will publish independent results when they exist. mylune is a wellness app for understanding your body, not a medical device.
The honest bottom line
If privacy is your priority, the decision is simpler than the listicles make it look. Choose an app where the data never leaves your phone. Today that means Euki, Apple's on-device tracking, or, when it launches, mylune. Everything on a server is a matter of trust, and 2026 showed how thin that trust can be.
Figures in this article are from Mozilla's July 2026 period tracker review and public reporting, and are current as of July 2026. Competitor practices change, so verify before you rely on any of them.