Jarrett Cooper raised an important and interesting question about the potentiality of fetuses to become adults in a comment, and I thought I'd make a post with a response. The center of the question is:
There are those who dislike the use of the potential verbiage that is used for pro-life arguments. Namely, to ground the moral worth of fetuses in that they have the potential (albeit undeveloped) to become beings with consciousnesses, intellect, language, etc. They argue that the word "potential" has a very broad scope and therefore defending the use of potential with regards to fetuses becomes arbitrary.
What I gather is that their concern is there are many things have potential to be such and such. After all, it is said that we are star dust, but yet we don't view exploding stars nor the mere star dust remnants themselves as human beings. This is because even if we are just star dust, there's a whole bunch of processes that have to occur to get a human being.
I think we can identify two relevant distinctions between how an F (say, a fetus, a bit of star dust, or some clay) becomes a G, and corresponding to these two distinctions there are relevant distinctions as to F's potentiality to become a G.
The first distinction is implicit in the rest of Mr. Cooper's comment and may be on a continuum. This is the distinction between F's changing itself into a G and F's being changed into a G.
For instance, an acorn changes or transforms into an oak tree, but an oak tree is changed or transformed into a canoe. In both cases there is both internal and external causal agency. The acorn needs water, soil and sunlight to become an oak tree. The oak tree needs cutting and joining to become a boat. Note, however, that the second of these two sentences, while having an interpretation on which it is true, sounds odd. For while the acorn seems to be primary agent when it becomes the oak tree, the boat-builder is the primary agent when the oak tree becomes a boat. The water, soil and sunlight have a supportive role in the transformation, and the acorn, driven by its DNA, has a primary active role. In the case of the boat, however, the boat-builder does not merely have a supportive role in the process. Either she is the primary agent, or else she and the oak tree are jointly primary agents.
Corresponding to this distinction there is a distinction between an F's potentiality to change itself into a G, and an F's potentiality to be changed into a G. Neither the human ovum nor star dust have the potentiality to change themselves themselves into an adult. In the case of the human ovum, that is because a genetically equal or almost equal input from the sperm is required (almost, since DNA may not be the sum total of the genetic code; cytological context matters, for instance). And a fortiori star dust can't change itself into an adult human.
There is a second distinction, perhaps more principled. There are three ways in which we can say that an F changes into a G. First, we have accidental change, where the entity that is initially identical with an F is eventually identical with a G. Thus, the child becomes the adult: the same individual that was the change is later the adult. Second, we have substantial change, where the entity that is an F perishes an a G comes into existence. For instance, this happens when a book burns into smoke and ashes. There is no one entity that once was a book and later is smoke and ashes (though there may be particles that once constituted a book and later constitute smoke and ashes). Third, we have what we might call constitutional change, where the F comes to constitute a G. In this case, the "be" in "F comes to be a G" is the "to be" of constitution rather than of identity. For instance, sticks become a house. But it is never literally true that the sticks are identical with a house. To be more precise, we should say that the sticks come to constitute the house.
It is never true to say that the entity that was identical with star dust is now identical with a human. What we can say, depending on difficult metaphysical questions, is at most that the star dust comes to constitute a human (or, better, a human's body) or that the particles that constituted star dust now constitute the human. Likewise, the ovum is not identical with the adult human. It changes substantially into a human adult: the ovum perishes, by merging with the sperm, and a human comes into existence from its death, like smoke and ashes come into existence from a book.
On the other hand, a fetus's change into an adult is an accidental change: the same entity that was identical with a fetus comes to be identical with an adult.
A potential to change oneself accidentally into an adult human is a much more morally significant potential than a potential to be changed substantially or constitutively into an adult human.
It may sound strange to say that a potential for an accidental change would give rise to a more important status than a potential for a substantial change. But the reason for that is that the fact that the fetus's change into an adults is an accidental change means that there is only an accidental difference between the fetus and an adult.