Your period tracker knows some of your most personal health details. The question is whether it keeps them private. Mozilla has published a new privacy review of six popular period tracking apps, ranking them from best to worst based on how they collect, store, and share sensitive reproductive health data.

One app earned a perfect score for keeping data on your device, while another landed at the bottom after researchers found it shared users’ health information with an analytics company. These findings arrive at a time when privacy around reproductive health data remains a growing concern, especially after the overturning of Roe v. Wade in the US.

Which period trackers did Mozilla rank best and worst?

Mozilla partnered with researchers from Harvard’s Berkman Klein Center and the University of Illinois to evaluate six apps through privacy policy reviews and technical testing. Here’s how they ranked:

• Euki (10/10): a nonprofit, open-source tracker that keeps everything stored locally on your device with no account required.
• Clue (8/10): a clinically minded German app that lets you separately control data sharing for research, analytics, and advertising.
• Flo (7/10): the most data-hungry tracker with an AI symptom chatbot, though testing found it kept health data away from advertisers.
• Period Calendar (6/10): the only ad-supported app tested, sending device details to Google and InMobi from the moment it opens.
• Spot On (5/10): Planned Parenthood’s tracker is safe within the app itself but leaky through its in-app browser for provider searches.
• Stardust (2/10): an astrology-themed app caught sharing birthdates, birth control methods, pregnancy status, reproductive goals, moods, and symptoms with analytics firm RudderStack.

Why did Stardust receive the lowest score?

Despite marketing itself with the tagline “Your data is private. Period,” Mozilla found Stardust sharing sensitive reproductive details with analytics company RudderStack. However, Stardust disputed that conclusion, telling BBC that RudderStack only serves as a secure pipeline into its analytics system and does not receive personally identifiable information.

Euki, meanwhile, earned its perfect score for a simple reason – there is almost nothing to leak in the first place since your data never leaves your phone. If you rely on a period tracker, Mozilla’s report suggests looking beyond marketing claims and remaining informed about how your data is collected, stored, and shared.