There's a much better argument to legalizing abortion, that holds even if one considers fetuses as persons. In that just as one may not be forced to donate organs or even just blood to save someone else's life, by the same logic, one cannot be forced to bear a child.
Of course this is an ethical, not legal argument, so I don't know how it would translate to US law.
Of course this is an ethical, not legal argument, so I don't know how it would translate to US law.