An adult human has the ability to look forward to their future and plan out what they would like it to become. A fetus, on the other hand, does not know what sort of experiences they could have, and they do not have the ability to plan out a future. As that is the case, a fetus is not aware that they are being deprived of anything when a pregnancy is terminated, the same way an adult is.
Another counter to the argument is that it allows abortions in too many cases. If a fetus cannot be expected to have a similar future due to a disability or because it will be born into horrible living conditions, then aborting the fetus may be acceptable according to this argument. That is not acceptable, because abortion should always be wrong.