The usual abortion dialectic, agreed to by both sides, is that the product of conception is either a person or not, and if it is a person, one cannot destroy it; but if it is not, they can. The second half of the disjunction is pretty clearly false- another man’s property is a clear case of something that is not a person, yet which we are not free to destroy. Even if we stipulate that some zygote is only a potential person, such a being is wholly understood in relation to the actual person who will arise, and so our actions with respect to it must be informed by its relation to the good of the person who will arise.
If the zygote is not a person, how are we to understand it? So far as it is relevant to this debate, it is either a part of another, or the property of another. Now anyone who imagines that the zygote is a part of its mother neither understands what “part” or “conception” mean, and while there is a real sense in which the zygote is the property of its parents, they can only be said to own it in the sense of owning it in trust- the way parents might oversee money (or a kingdom) that is to be given to a child when he comes of age. The zygote’s whole nature- genetic code, sex difference, life, growth, etc- is a nature ordered to being given to another, like money held in trust.