A trial in the closely watched class-action lawsuit accusing a period tracking app of improperly sharing user data with Meta for targeted advertising opened in a Northern California federal court on Monday.

Meta and Flo Health are co-defendants in the case, which could reportedly lead to billions of dollars in damages since the plaintiffs say there are 38 million members of the class.

The lawsuit alleges that despite promising women it would not share their private sexual and reproductive data with third parties, Flo allowed Meta to embed a software development kit in the app which let it harvest data about women' s periods.

Meta lawyers on Monday reportedly denied that it obtained the menstruation data from Flo.

The lawsuit was filed in the wake of a 2019 Wall Street Journal story reporting that, despite its promises of confidentiality, Flo shared users’ data with Meta, Google and other third parties who used it for targeted advertising.

In June 2021, the Federal Trade Commission finalized a settlement with Flo which required it to notify users impacted by data sharing of the disclosures and to get users’ affirmative consent before sharing their sensitive health data in the future.