She's not facing 18 months. They are pressing some small charges against her and some other protestors, and the max prison time could be 18 months.

Every news article I’ve ever read that contained the phrase ‘facing [x] months in prison’ chose the maximum possible sentence for x. That’s just what that phrase is meant to convey, that the law allows for a punishment as high as [x] months.

Example: https://www.yahoo.com/news/politics/articles/congresswoman-8-months-pregnant-facing-120000922.html