Such a thing does not exist. Something is either a human being or it is not. Adding “potential” as a qualifier to dehumanize the unborn just helps people rationalize the evil of abortion, much like Jews were dehumanized in Nazi Germany and still are in some Arab cultures today, and how blacks were dehumanized in the U.S.
If pro-choicers want to rationalize that the unborn are just potential humans, then they can just have “potential abortions.”
The definition of personhood usually involves some reference to self-awareness. Not only is this definition hard to pin down, it is arbitrary. Who got to decide that this was the paramount life standard? In theory, it would put us all at risk. Do you have self-awareness when you sleep? What if someone is in a coma? (Oops, bad example). Personhood is a philosophical notion, not a scientific one, so anyone basing their views purely on science shouldn’t be using it.
There is also no consensus on the definition, which gives a big hint that perhaps we should not be using this man-made criterion to determine whether “something” should be able to be destroyed or not. Peter Singer, a professional “ethicist,” thinks you should be able to kill babies if that will maximize everyone’s theoretical happiness (except the baby’s, of course).
Note how those in power get to decide who is a person. When one group gets to decide whether another group has personhood, bad things happen (slavery, the Holocaust, genocide, abortion, disabled people, etc.). The reasoning in the Dred Scott case (that Blacks weren’t persons) is eerily similar to the personhood reasoning put forth by pro-choicers.
The unborn are human beings. Just because they are smaller, less developed, dependent on others and are in a different environment doesn’t mean they are fair game to be destroyed.
Update: Right after this posted I came across an article titled Activists Want Chimp Declared a ‘Person.’ Really, you can’t make this stuff up.
If you aren’t sure when life begins, you should err on the side of life.