I'm torn, to be honest. I don't like the idea of government forcing anyone to do anything. This extends beyond just what they can do with their body, but also their time, money, etc. In this case, as well as pregnant women, A and B have a symbiotic relationship. How do you define when one person's body ends and another starts?
Also, starting down the road of "the government can't force anyone to do anything with their body" is a dangerous road of logic. Can the government intervene if someone is trying to commit suicide? Require immunizations to enroll in school? Prohibit physical altercations between two willing participants? Wear a seat belt? I don't have the answer of when it's okay for the government to intervene. I think most people would say that in some instances it is appropriate.