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.