While the fetus may not be able to access the rational capacities of a grown adult, it will eventually be able to access those capabilities. The distinction is akin to that of latent versus actualized capacities.
Someone talking in a conversation is actualizing a capacity for speech. The same person may be capable of learning to play the piano if they chose to take lessons. The capacity to learn the piano might be compared to the capacity of a fetus to be a rational, self-aware adult. Neither one of those things have been achieved, but the capacity is there.