The Mozilla Foundation's investigation into six popular period trackers—Flo, Clue, Stardust, Spot On, Period Calendar, and Euki—reveals a stark divide in privacy practices. Stardust was found to share detailed reproductive health data, including pregnancy status and birth control use, with data management company RudderStack, a practice not disclosed in its privacy policy. While Stardust claims this is merely a technical pipeline and that RudderStack cannot identify users, experts warn that spreading data to more parties increases risks of breaches or legal requests. In contrast, Euki emerged as the only app recommended without reservations, storing all health data locally on the user's device and offering a decoy feature for added security.