First let me state, I have not read this book, if you use terms from it, I will not know what you are talking about.
Spencer is making a strawman arguement as that arguement in that form is not likely made by many prochoicers. Especially as their arguement in complete.
Let me make a few arguements for abortion choice before two weeks.
Murder is the act of killing a sentient being. Or a person. (alot of fluidity in the definition of murder in this area)
AFAIK before 3 days the cells have yet to differentiate. That means they are exactly the same. A glob of cells with the only difference between them and a similar glob of cells of a mouse at the same stage is DNA and perhaps size.
AFAIK before two weeks there isn’t a single nerve cell. We are our thoughts. Our brains make us, us. Without a working brain, we are essentially dead. A brain is a complex bundle of nerves. Without a single nerve, we have no mind, we are not “alive” as a person yet.
Since it is brain dead and not alive as a person yet, it is not murder to destroy it.
If you speak of what it might become, then not doing it like bunny rabbits without condoms with every non pregnant girl of child bearing years you meet even if shes just a child yet and doesn’t want to, could potentially be construed as murder. Because every moment a girl is not pregnant when she could be is one more human life that could be, that isn’t. That is where such a reasoning of “future person” leads us.