I don't think those two issues are entirely different.
If you think that life only begins at successful, sustainable breathing of a baby outside the womb, then until that point, the 'baby' is only a 'potential baby' hosted by the woman (or child in some cases).
It follows that the 'host' is able to decide how long she 'hosts' for and therefore can choose to stop hosting the potential baby and prevent it from becoming a 'baby'.
How far along toward the point at which 'life' would be expected to come from the birth process, as opposed to breathing dependant on machinery, is a matter for contention and debate.
Obviously, if you think that life begins at conception, then there are different considerations.
But the question of whether the rules should be different for mothers found to be carrying a disabled child are necessarily a follow-on from the first issue. If someone ascribes to the first view of 'life' then they are going to have a similar view regarding disability, surely? I mean, until the point of birth, there is simply a 'potential baby' and therefore, disabled or not, the choice remains to terminate.
The real grey area begins when people hold a view that it is unreasonable to abort past a certain gestation except for disability, and that is what we were looking at originally.
Then you have to look at whether the difference is an ideological one, or one driven by practicalities. Perhaps if all disabilities could be detected earlier, there would be no distinction between the two. But the case is that we can't detect all serious disabilities earlier.
That aside, it is interesting how we all know that many children who are disabled are healthy, yet we manage to confuse the two when we talk about the rationale for abortion, even if we have acknowledged the distinction earlier in discussions.