Menstruation tracking apps have been sharing highly sensitive data with Facebook Inc., the U.K.-based advocacy group Privacy International said Monday.

According to that group’s research, apps such as Maya by Plackal Tech and MIA by Mobapp Development Limited shared information with Facebook about menstruation cycles, menstruation symptoms, mood and general health, but also the use of contraception and sexual activity.

In a post, Privacy International said that though these apps exist for the most part only to tell a woman when she’s most fertile or when her next period is due, users in fact share information relating to health and also sexual activity.

The data is collected through Facebook’s Software Development Kit, and in turn developers can better understand how the app is used while Facebook can use the information to target people with ads. In the case of Maya, which has more than 5 million downloads on Google Play, the app informs Facebook when it is opened.

“There is already a lot of information Facebook can assume from that simple notification: that you are probably a woman, probably menstruating, possibly trying to have (or trying to avoid having) a baby,” said Privacy International. “Moreover, even though you are asked to agree to their privacy policy, Maya starts sharing data with Facebook before you get to agree to anything. This raises some serious transparency concerns.”

On top of that, users of the app can input health data relating to how they feel on a certain day. Privacy International said this information is also shared with Facebook. This could be simply the user saying they have a headache or more acne, or what mood they are in that day, but the app goes further and can share information about the last time the person used contraception.

All this personal information about a person’s well-being and mood, said Privacy International, is a treasure trove for advertisers. Users of the app are also asked to write detailed information about how they feel, and this is also shared.

The same goes for MIA, which has about 2 million users. Once the app is opened, Facebook is informed, which already says a lot about the person using it. MIA doesn’t share the information directly, but it targets the user with articles. Once they are opened, that information is sent to Facebook and a third-party analytics company.

The research also found other similar apps to be sharing sensitive personal information.

Image: Dafne Cholet/Flickr