I think they also --deliberately-confuse legal personhood with the person. Positivism allows us to define and redefine things to suit our needs. Legal personhood IS something that the law can create: one can arbitrarily say that a corporation is a person because it is useful to say this. Our system allows the courts the power to vest meanings in words, meanings that sometimes contract the evidence of our senses. The court says that a child of human being is not a human simply because it is useful for the child not to be human. Talk about priestcraft!
It's not a human life. It's part of the mother. It's not alive. Etc.
Pro-life person proves it is human, is alive, isn't part of the mother, and so on.
Well, the woman has a right to abort it anyway, because it's a parasite. (or some variation of that theme)
Pro-life person demonstrates other examples of parental responsibility and so forth, based on the already proven premise that it's the equivalent of a baby.
But it's not a human life. It's part of the mother. It's not alive. Etc.
They never actually concede a point and simply recycle the same arguments round and round in a big circle. They do the same thing with life, personhood, etc. They can waste a whole lot of time on silly tangents like neuron growth, myleanation, etc. that ultimately have nothing to do with personhood. The reality is that many people pick a point where they want the embryo or fetus to be a person and then work their way into an excuse to make it true.
The way around that is to stick to small, atomic, easy to defend steps and then snap them all together into a tight package that they can't knock down at the end. Once you understand all the small parts and the role they play in the whole, it becomes easy to spot when they try to change the subject, avoid a point, and so on. Making sweeping statements are good for an overview but not very good at changing minds, in my opinion. In fact, one thing I despise about the modern talk radio and talk television formats is that they confine arguments to maybe 10 minutes but often closer to 2 minutes and you can't make a comprehensive and persuasive argument about abortion in that short of a period of time. The best you can do is preach to the choir.