Popular period tracking apps are facing renewed scrutiny over how they collect, protect, and share users’ sensitive health information.

A new report from the Mozilla Foundation examined the privacy practices of six widely used period tracking apps: Flo, Clue, Stardust, Spot On, Period Calendar, and Euki.

The investigation found that privacy protections and data practices can vary significantly between apps, despite some services making strong promises about keeping users’ information private.

Stardust, for example, promotes itself as a privacy-focused period tracker and tells users that their data is private. However, the new research suggests that users and companies may have very different interpretations of what “private” actually means.

Period tracking apps can collect highly sensitive information, including menstrual cycle details and other health-related data. Depending on an app’s privacy policies and data practices, information may potentially be shared with third parties or used for other purposes.

The findings highlight the importance of carefully reviewing an app’s privacy policies before entering sensitive personal health information.

For users concerned about digital privacy, understanding how period tracker apps handle data can be an important step in protecting personal health information online.