The sharing happens through the Facebook Software Development Kit (SDK), a set of software development tools that can be used to develop apps for a specific operating system.

Menstruation apps collect information about a woman’s health, sex life, mood and more – all in exchange for telling you what day of the month you’re most fertile or the date of your next period.

Privacy International looked at apps including Period Tracker by Leap Fitness Group; Period Tracker Flo by Flo Health, Inc.; Period Tracker by Simple Design Ltd.; and Clue Period Tracker by Biowink who was called out on the practice last year. None of them shared data with Facebook.

However, Maya by Plackal Tech, MIA by Mobapp Development Limited, My Period Tracker by Linchpin Health did change its mind After Privacy international shared this report: 

“We understand your concern that in addition to providing the analytics SDK, Facebook is also a social network and an ad network. We have hence removed both the Facebook core SDK and Analytics SDK from Maya. Version 3.6.7.7 with these changes is live on the Google Play Store and will be submitted for review to the Apple App Store by this weekend. We continue to use the Facebook Ad SDK, post opt-in to our terms and conditions and privacy policy. Maya does not share any personally identifiable data or medical data with the Facebook Ad SDK. The Ad SDK helps us earn revenue by displaying ads that our users can opt-out of by subscribing to Maya's premium subscription.”

Linchpin Health did not respond, and MIA did not wish to have their response published.

Privacy International points out that there are good reasons why advertisers want to know your mood. At a banal level, they will pitch certain food types at women at certain points in their cycle or hit you with PMT remedies at another.