The irrational argument is aesthetic and emotional and instinctive, I suppose. A foetus at 15 weeks doesn't look like a baby, while one at 40 weeks does - because it is. A more rational argument, however, is viability. If a child is viable outside the womb and you kill it, there can be no mistake that you are taking the life of a human being whose right to life ought to be vouchsafed by the law.
In the UK, abortion is legal up to 24 weeks, yet babies are viable up to and even before this gestation period. So should I assume that you think the current laws allow "the taking the life of a human being whose right to life ought to be vouchsafed by the law" and are therefore wrong?
Obviously an arbitrary limit must be set and in this case it's 24 weeks. Survivability up to that date is very low and the kid will probably be pretty fücked even if it makes it. If you have to set a limit, then I agree that the general principle of viability is how you have to set it and that 24 weeks is reasonable - even if there are exceptions who make it at that age. Maybe I'd go a week or two lower, but I'm understanding of the legislators' reasons for setting it where they do.
That's about right. Abortion should not be a form of contraception but the life of the mother must take absolute paramountcy over the life of the unborn foetus. Severely handicapped and product of rape and so forth should be aborted whenever, no limits. The religious crap about abortion is driven more by their desire to overpopulate the world so there's plenty of cash for them in this world and souls in the next and not due to the sanctity of life - they have been happy to behead, stone, burn and otherwise extinguish life over the centuries when it suits them. All of them.