Pro-lifers ought not in principle make any distinctions between early and later abortions, yet it is plain that they, like other people, often do. There seems to be some confirmation to the gradualist or developmentalist view. The further along the path towards birth, the more protection we think the human fetus should have, or the more serious the reason must be for abortion. 1)
We can accept Warnock's intuition that later abortion is more problematic than earlier abortions, but not accept the grounds for this that the human fetus gains moral status throughout development, i.e. accepting the intuition without accepting the gradualist view.
I think Ainslie wanted to argue a gradualist view until some magical plateau is reached (or until there's enough of a rope) somewhere in the middle of pregnancy, and would side-step pre-term delivery by saying perhaps there isn't proportionate reason to kill the child once born because the issue of a mother's bodily autonomy is off the table (rather than weighed against a gradually developing right to life)