Someone else mentioned this thread today (but didn't post) here, for some reason.
This happened to me as well. In case this happens to anyone else - in my case, it was a billing error; the hospital coded the miscarriage (and subsequent tests, I had some minor complications) as an elective abortion, which is not covered under my plan. I called the insurance company, and within minutes they called my doctor and the hospital and confirmed it was an error. So, if this happens to you, check how the hospital coded the procedures.