I read the headline thinking they were using digital medical data, i.e., what insurance companies do. That would be legitimately frightening.
Nope, police are just exploiting bad opsec. Don't bring your phone to your robbery, don't talk about your alleged crimes on Facebook Messenger. Lots of people don't know that these channels are not secure.
(Edit: Abortion is currently illegal in some U.S. jurisdictions. There is debate about whether it should or shouldn't be.)