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Period-tracking apps used by millions of women are sharing information about their sex lives and health with Facebook, a new study revealed.

Researchers with the advocacy group Privacy International published a report Monday that found two period-tracking apps, MIA Fem and Maya, sent data regarding users' health to the social media giant.

"When Maya asks you to enter how you feel and offers suggestions of symptoms you might have — suggestions like blood pressure, swelling or acne — one would hope this data would be treated with extra care," the study said. "But no, that information is shared with Facebook."

Both Maya and MIA Fem have sent information on users' contraception, the timing of their periods, symptoms during cycles and even the last time they had sex, to Facebook through the social media giant's Software Development Kit, which helps the site target ads, according to the study.

But Maya's parent company, Plackal Tech, has denied that any personally identifiable or medical data gets funneled to Facebook.

The company said the app, which has more than 5 million Google Play store downloads, uses the Software Development Kit to "earn revenue by displaying ads that our users can opt out of by subscribing to Maya's premium subscription," the advocacy group reported.

MIA Fem — which has more than 1 million users — didn't immediately respond to request for comment.

In response to the study's findings, Facebook said that it prohibits app developers from sharing customer data about health, finances or "other categories of sensitive information."

"We have systems in place to detect and delete certain types of data such as Social Security Numbers, passwords, and other personal data, such as email or phone number," a Facebook spokesperson told Privacy International. "We have begun looking at ways to improve our system and products to detect and filter out more types of potentially sensitive data."