For me, the whole argument goes haywire because any discussion / decision about abortion that is based on whether the feotus is one (in particular) that the parents want immediately rests on value judgements about people with disabilities.
However, in theory, I actually believe that the abortion argument should be about whether a woman wants a feotus developing in her body or not. Any feotus. Whether, in short, she wants to be pregnant, and be a mother. If not - I think she should have the ight to abort. And if the law says 39 weeks for some pregnancies - then it should be for all pregnancies.
Viscerally, of course, the idea of terminating a foetus - any feotus - at 39 weeks is unthinkable. And because of that, I doubt many people do it. Well, they don't do they? the figures are tiny. I wonder, in truth, whether abortions carried out late due to 'disbility' of the cleft palate / talipes variety are actually women wh simply do not want to be pg, to have any baby, and manage to use the disability get-out? And to be honest, if that is the case, m own view would be 'let them have an abortion, and don't make them use disability as a reason'. DS's condition was identified as talipes at 20 week, and it would have been beyond preposterous to terminate on that basis - I honestly can't imagine a woman who wanted a baby considering termination at that stag fo something so minor...unless she actually didn't want to be pg at all.
So - I think there is a terrible conflation (caused by the discrepancy for disability) between abortion per se, and disability.
If abortion is allowed, it should be because it is allowed on the woman's say-so - because it is her body. And probably up until birth - ghastly though that is in practice. But it wouldn't happen v often. Probably not more often than it happens now.
Of course, some women would perhaps still choose that option once they discovered disability...but the option would be there because it is a woman's choice, NOT dependent on the feotus' ability.