Well, there's the issue of bodily autonomy, and the fact that we can't even force dead people to donate their organs to save lives. There's also a difference between being pregnant and witnessing someone on the street dying - the former will affect your body to a far greater extent than the latter. And to go back what I said above - if not even dead people can have their bodily autonomy broken why should pregnant women's autonomy be? No harm or discomfort will come to that dead person, yet the woman should be made to suffer just because? The problem with analogies is that they often aren't 1:1, and when picked apart you will find differences between them. Even your original argument can be said to have this problem, and it all just depends on how nit-picky and semantic people want to be about it.
I think a lot of the problems with the discussion stems from differing views on what life is. At best you could try an argument that they have no right to force their beliefs, on when personhood begins, on another human being. "Pro-lifers" often derive their rationalization from religion, or traditionalism, sometimes even misandry. Life can be defined in many ways, and while the scientific definition of it would apply to a day old embryo it would also apply to your skin cells. "Pro-lifers" sometimes use this as an argument, that science says that this is life but then ignore the other implications of that fact or reframe it themselves as two unique DNA's combining into new life (making the scientific basis of their argument disappear).