Your cycle observations are stored on your device.
Here's what that means.

~6 min read

When I started building Wren, I had to make a decision that scared me. I'm going to explain exactly what I decided and why — because you deserve to know.

I'm one person building this

I'm 23. I'm building Wren alone. I'm learning as I go. And at some point while building the cycle tracking feature, I had a moment of genuine fear — because I realised I was asking women to hand over some of the most sensitive information that exists about their bodies.

Period data isn't just personal. In the current legal climate, it can be consequential. It can be subpoenaed. It can be used in ways none of us want to think about. I thought about that for a long time before I wrote a single line of code for this feature.

The decision

Cycle observations you log in Wren are stored in the app on your device, not on Wren's servers.

This includes period start and end, spotting, symptoms you log, your tracking preference, cycle type, typical cycle length, and last-period anchor. Wren may use symptoms and confirmed bleeding on your device to support Today's capacity and recovery guidance. During beta, estimated phase does not change nutrition targets, Fuel Score, training, Next Best Move, Coach responses, or menus.

What stays where

The trade-off I'm making

Device-local storage means cycle history does not sync between devices. Reinstalling Wren or clearing app storage removes it. Signing out leaves it on that device, isolated to the account that created it; deleting the account removes Wren-managed local cycle information for that account. Device or operating-system backups may include app data under the backup provider's controls.

It also means I learn less about how cycle tracking is used. During beta, Wren measures only whether tracking is enabled and one later-return milestone. We do not collect per-log events, symptom types, bleeding, spotting, period dates, cycle day, or estimated phase for analytics.

The science behind the recommendations

Wren's general nutrition, training, and recovery guidance draws on published research. Deeper cycle-aware personalization is intentionally deferred until the model and intended product use receive appropriate health review.

I'm not a doctor. Wren is not medical advice. But the foundation it's built on is science.

Why I'm telling you this

Because I think you should know. Because I'd want to know. And because the kind of app I want to build is one where the founder can look every user in the eye and explain every decision she made.

This is that explanation.

Wren · From the Founder · Questions or feedback? Reach us at hello@bellamatasic.com