I would be interested in hearing from pro-choicers as to whether they agree that the manslaughter charge is appropriate in this case.
As a pro-choice supporter, the question of whether the charge is appropriate or not doesn't matter too much (to me).
I lean Libertarian in some ways, largely favoring states' rights when it comes to contentious issues like marriage, abortion, etc. In this particular case, I think states can/should craft manslaughter laws as they see fit (within the confines of the constitution and other legal standards, etc).
In other words, if a state says that causing the death of a pregnant woman's fetus without her consent equates to manslaughter, I'm fine with that. I may not agree, but my (dis)agreement is moot - because the state has the right to craft this definition/law.
ps. kudos for asking your question respectfully. There are many here (including the thread's author) who seek to politicize issues with the aim of insulting those he/she/it disagrees with. By contrast, you asked the questions of "pro-choicers", rather than liberals or Democrats or whatever political label that can be used to dismiss the opinions of the people the question is being asked of. In other words, your question was obviously sincere, which was refreshing change of pace.