It is certainly alive, and it is certainly human. There is nothing potential about these facts. However, if you wish to define something like “personhood,” or to explain how the rights a human life deserves are somehow derived from the being’s current physical state, or to explain why the being’s rights can be morally subjugated to someone else’s higher rights–well, provide your rationale.
“Human life deserves protection, but this isn’t a human life” fails at the start.
To specifically answer the OP:
According to my axioms (YMMV;)), after conception, none of us have ever existed in a form that was not essentially human. Different physical attributes at different phases, certainly. But always human. I also hold as axiomatic that any real being possessing even the potential for sentience is a “person.” When that potential no longer exists, there is no longer a person.
In those rare instances where a brain dead patient recovers, for example, I do not believe that during the period of “brain death” we could morally kill this being, at least not if we knew the potential for recovery existed. A brain dead person who will recover is a person. A fetus (who clearly possesses the potential for sentience) is a person. A corpse is not. FWIW, this is my moral boundary around human rights that deserve protection, whatever those rights might be.
I also believe that a person’s right to live is the most fundamental right, even if it cannot be protected absolutely (for example, what if my right to live conflicts with your right to live?). All other rights I can conceive of are meaningless if the right to live is not at their foundation.
You may argue that one has the right to self determination, the right to purge one’s body of an “embryonic parasite,” the right to engage in sexual activity freely without fear of certain consequences, etc.–and those rights may be absolutely real and important–but those rights have little value if there is no moral restriction against my killing the “owner” of the right. They must be lesser rights because from a practical perspective they don’t exist without the fundamental right to live.
I also hold that this can’t be true only for my rights–meaning, I don’t just hold that my “other” rights are less important than my right to live, while simultaneously being more important than any right someone else might possess. If I allowed this, than I’d have to allow this as true for others, which would effectively negate my right to live. Therefore, the right to live is a fundamental–the most fundamental–right. If there is such a thing as an absolute right, this is it. If there is such a thing as the most “horrendous sin,” to use AHunter3’s term, it is the violation of this right.
So, no, in my worldview, I could not morally find a rationale for abortion (except, perhaps, in rare instances where the mother’s life would be lost–though I have read some articles that suggest this is rare to the point of being non-existent).
I also know and love people who are pro-choice, people I consider moral human beings overall (none of us are morally “pure”). I do believe, though, that those people are confused on this issue, even if it seems crystal clear in their own minds.