Well on iOS there’s the Apple health app. To my knowledge it stores health data locally. I’ll double check now.

Edit: it does store health data in iCloud by default, but according to Apple its end to end encrypted

By default, iCloud automatically keeps your Health app data, including health records, up to date across your devices. To disable this feature, open iCloud settings and turn off Health. iCloud protects your health records data by encrypting it both in storage and during transmission. If you're using iOS 12 or later and have turned on two-factor authentication for your Apple Account, health records are encrypted using end-to-end encryption through iCloud. This means only you can access this information, and only on devices where you’re signed in to iCloud. No one else, not even Apple, can access end-to-end encrypted information.

Cambridge researchers urge public health bodies like the NHS to provide trustworthy, research-driven alternatives to platforms driven by profit.

Women deserve better than to have their menstrual tracking data treated as consumer data - Prof Gina Neff

Smartphone apps that track menstrual cycles are a “gold mine” for consumer profiling, collecting information on everything from exercise, diet and medication to sexual preferences, hormone levels and contraception use.

This is according to a new report from the University of Cambridge’s Minderoo Centre for Technology and Democracy, which argues that the financial worth of this data is “vastly underestimated” by users who supply profit-driven companies with highly intimate details in a market lacking in regulation.

The report’s authors caution that cycle tracking app (CTA) data in the wrong hands could result in risks to job prospects, workplace monitoring, health insurance discrimination and cyberstalking – and limit access to abortion.

They call for better governance of the booming ‘femtech’ industry to protect users when their data is sold at scale, arguing that apps must provide clear consent options rather than all-or-nothing data collection, and urge public health bodies to launch alternatives to commercial CTAs.

Menstrual tracking app data is a ‘gold mine’ for advertisers that risks women’s safety – report

Smartphone apps that track menstrual cycles are a ‘gold mine’ for consumer profiling, collecting information on everything from exercise, diet and medication

University of Cambridge (www.cam.ac.uk)

No, we’re simply using the same logic as you and not letting perfect be the enemy of good. Telling people to use the built in app on the most popular platform because it has infinitely better privacy than all the apps with ads is objectively good advice. My grandma doesn’t know what f-droid is. She doesn’t even know what an operating system is. She also doesn’t know what a software license is.

Cambridge researchers urge public health bodies like the NHS to provide trustworthy, research-driven alternatives to platforms driven by profit.

Women deserve better than to have their menstrual tracking data treated as consumer data - Prof Gina Neff

Smartphone apps that track menstrual cycles are a “gold mine” for consumer profiling, collecting information on everything from exercise, diet and medication to sexual preferences, hormone levels and contraception use.

This is according to a new report from the University of Cambridge’s Minderoo Centre for Technology and Democracy, which argues that the financial worth of this data is “vastly underestimated” by users who supply profit-driven companies with highly intimate details in a market lacking in regulation.

The report’s authors caution that cycle tracking app (CTA) data in the wrong hands could result in risks to job prospects, workplace monitoring, health insurance discrimination and cyberstalking – and limit access to abortion.

They call for better governance of the booming ‘femtech’ industry to protect users when their data is sold at scale, arguing that apps must provide clear consent options rather than all-or-nothing data collection, and urge public health bodies to launch alternatives to commercial CTAs.

Menstrual tracking app data is a ‘gold mine’ for advertisers that risks women’s safety – report

Smartphone apps that track menstrual cycles are a ‘gold mine’ for consumer profiling, collecting information on everything from exercise, diet and medication

University of Cambridge (www.cam.ac.uk)

Wrong, Apple Health fails to include a libre software license text file. We do not control it, anti-libre software. Does Apple really think we are this easy to scam? It bans us from fixing backdoors.

I get that. It shouldn't be necessary to do research just to be sure it's safe. I apologise for my aggressive tone. It must be really scary and exhausting to live somewhere like that. My country is still holding up but seems to be sliding slowly in that direction too.

So do whatever makes you stay safe and feel safe.

Drip (Android/iOS)

I mean, sure - but if you really don't trust Apple to keep their word, then it wouldn't matter if their Health app was FOSS or not. iOS itself is still (and probably forever will be) a closed source operating system. That gives them the power to do anything, including hijack the data from FOSS apps.

Do they include period prediction now?

Cambridge researchers urge public health bodies like the NHS to provide trustworthy, research-driven alternatives to platforms driven by profit.

Women deserve better than to have their menstrual tracking data treated as consumer data - Prof Gina Neff

Smartphone apps that track menstrual cycles are a “gold mine” for consumer profiling, collecting information on everything from exercise, diet and medication to sexual preferences, hormone levels and contraception use.

This is according to a new report from the University of Cambridge’s Minderoo Centre for Technology and Democracy, which argues that the financial worth of this data is “vastly underestimated” by users who supply profit-driven companies with highly intimate details in a market lacking in regulation.

The report’s authors caution that cycle tracking app (CTA) data in the wrong hands could result in risks to job prospects, workplace monitoring, health insurance discrimination and cyberstalking – and limit access to abortion.

They call for better governance of the booming ‘femtech’ industry to protect users when their data is sold at scale, arguing that apps must provide clear consent options rather than all-or-nothing data collection, and urge public health bodies to launch alternatives to commercial CTAs.

Menstrual tracking app data is a ‘gold mine’ for advertisers that risks women’s safety – report

Smartphone apps that track menstrual cycles are a ‘gold mine’ for consumer profiling, collecting information on everything from exercise, diet and medication

University of Cambridge (www.cam.ac.uk)

Lol sure apple doesnt use that data internally. /s

Cambridge researchers urge public health bodies like the NHS to provide trustworthy, research-driven alternatives to platforms driven by profit.

Women deserve better than to have their menstrual tracking data treated as consumer data - Prof Gina Neff

Smartphone apps that track menstrual cycles are a “gold mine” for consumer profiling, collecting information on everything from exercise, diet and medication to sexual preferences, hormone levels and contraception use.

This is according to a new report from the University of Cambridge’s Minderoo Centre for Technology and Democracy, which argues that the financial worth of this data is “vastly underestimated” by users who supply profit-driven companies with highly intimate details in a market lacking in regulation.

The report’s authors caution that cycle tracking app (CTA) data in the wrong hands could result in risks to job prospects, workplace monitoring, health insurance discrimination and cyberstalking – and limit access to abortion.

They call for better governance of the booming ‘femtech’ industry to protect users when their data is sold at scale, arguing that apps must provide clear consent options rather than all-or-nothing data collection, and urge public health bodies to launch alternatives to commercial CTAs.

Menstrual tracking app data is a ‘gold mine’ for advertisers that risks women’s safety – report

Smartphone apps that track menstrual cycles are a ‘gold mine’ for consumer profiling, collecting information on everything from exercise, diet and medication

University of Cambridge (www.cam.ac.uk)

You need to teach me this skill. Any excuse to get frisky with my wife.

There is!
It's called drip and is a project started by a berlin-based feminist collective iirc.

drip. app

drip. menstrual cycle and fertility tracking

(dripapp.org)

Free, open-source, local data only

Cambridge researchers urge public health bodies like the NHS to provide trustworthy, research-driven alternatives to platforms driven by profit.

Women deserve better than to have their menstrual tracking data treated as consumer data - Prof Gina Neff

Smartphone apps that track menstrual cycles are a “gold mine” for consumer profiling, collecting information on everything from exercise, diet and medication to sexual preferences, hormone levels and contraception use.

This is according to a new report from the University of Cambridge’s Minderoo Centre for Technology and Democracy, which argues that the financial worth of this data is “vastly underestimated” by users who supply profit-driven companies with highly intimate details in a market lacking in regulation.

The report’s authors caution that cycle tracking app (CTA) data in the wrong hands could result in risks to job prospects, workplace monitoring, health insurance discrimination and cyberstalking – and limit access to abortion.

They call for better governance of the booming ‘femtech’ industry to protect users when their data is sold at scale, arguing that apps must provide clear consent options rather than all-or-nothing data collection, and urge public health bodies to launch alternatives to commercial CTAs.

Menstrual tracking app data is a ‘gold mine’ for advertisers that risks women’s safety – report

Smartphone apps that track menstrual cycles are a ‘gold mine’ for consumer profiling, collecting information on everything from exercise, diet and medication

University of Cambridge (www.cam.ac.uk)

The humans in my family who experience menstrual cycles have been pretty happy with Clue who have an explicit promise to never give up your data. YMMV and of course you should evaluate what a promise from this organization means to you.

Yes of course but it's marketing data.

Marketing data. We need it for marketing to people so they can spend money.

Don't you understand! Marketing data!!!

I'm not sure what the best answer to that is. I don't think it's forcing Google to improve its search results.

I want it to be the average person gaining a baseline level of computer and media literacy such that they seek out and find apps that cannot send sensitive data to third parties without the user's clear intent, but I don't think we'll ever get there.

It's not nefarious! We just wanna send you the right ads when your hormones are in flux and you're vulnerable!

Also we promise we definitely will hand over your menstrual data the second your GOP Governor requests it. /s

If we're gonna be going full cyberpunk I'd better see a lot more fuckin neon over the next few years.

Made my own desktop app in python (tkinter) which encrypts the data with GPG. It has predictions and potential ovulation days. The predictions seem pretty accurate so far.