If we're talking about personal opinions, mine is that abortion in the first few weeks after conception doesn't present a moral problem, for the reasons Daverupa mentioned. And I also think it is justifiable when the mother's health is at stake.
However, my view is informed by secular ethics rather than Theravada moral norms, and I believe Will was asking an informational question about what (Theravada) Buddhism teaches. It seems difficult, to me, to find a basis in the suttas for justifying abortion at any stage of pregnancy, except when the mother's life is in danger. Again, let me stress that I'm simply trying to answer Will's question, not promulgate my own views.
The issue seems entangled fairly deeply with everyone's favorite dhamma topic, rebirth. If we go by the orthodox Theravada teachings, which not only assert that rebirth takes place but provide a detailed outline of how it takes place, then abortion clearly violates the first precept. Worth remembering here, perhaps, that even intentional killing of a bumblebee is a breach of the precept, at least from an orthodox point of view.
Unfortunately, if one is agnostic about rebirth, there is still a potential moral problem. Being agnostic means you accept the possibility that rebirth might be true, in which case abortion isn't ahimsa.
If one rejects rebirth, then we 're left with the need to define a starting point for "life" in an ethically meaningful sense. Maybe the viability threshold.
Last edited by Lazy_eye
on Sat May 11, 2013 6:58 pm, edited 2 times in total.