Traditionally, a rational will; i.e., a sufficiently free will. That is a serious and impactful difference between humans and other species: most, if not all, other species lack the capacity to go against their own nature and inclinations such that they are motivated by pure reason.

Traditionally, a being which has a Telos such that it will have, if not already has, a rational will are called persons (because their nature marks them out to be such); and their will must be respected.

More technically, a being which has a such a "rational Telos" is not necessarily a person but, rather, will be; and their nature marks them out as such; and this is what grounds their rights (and not whether or not they currently are a person). — Bob Ross

Strangely, you seem to be saying that the nature (capacity for reason and abstract goals) which allows abortion is what grounds the right to not be aborted.