A Mozilla Foundation audit in partnership with Harvard's Berkman Klein Center examined privacy practices across six popular period tracking applications and uncovered systematic data harvesting by commercial developers. The worst performer, Stardust, transmitted users' reproductive health records—including birth control methods, pregnancy status, and specific symptoms—to undisclosed third-party firms beginning at app launch before users entered any information. The application routed data through analytics platforms with no way for users to disable sharing and also connected user behaviour to Facebook's advertising profiles using persistent identifiers.

By contrast, Euki, a nonprofit-operated tracker, achieved perfect privacy marks by storing all health information exclusively on users' devices, requiring no account, and offering controls such as PIN protection and scheduled data deletion. The audit highlighted a fundamental divide in how reproductive health applications treat sensitive personal information, with most commercial services optimising for monetisation through data partnerships rather than user privacy. The findings raise questions about consent and the collection of intimate data by apps targeting women's health.