At what stage in the development of a human being does it cease to be morally permissible to destroy it?

Abortion and Infanticide

One reason the question of the morality of infanticide is worth examining is that it seems very
difficult to formulate a completely satisfactory liberal position on abortion without coming to grips with
the infanticide issue.

The problem the liberal encounters is essentially that of specifying a cutoff point which is not arbitrary: At what stage in the development of a human being does it cease to be morally permissible to destroy it?