Abortion: No Bright Line
I quote here comments I left at The Debate Link in resonse to an interesting set of thought experiments: things along the lines of 'whom would you save from a burning building--zygotes or a 4 year old? an adult or a 4 year old? a premie or a 4 year old? etc.
The structure of all the hypotheticals you give is an exaggerated choice of passively allowing one life or group of lives to end in order to save others. The first one is designed, it seems, to make visible the 'bright line' between human life and non- or proto- life. The problems that reducing the exercise to absurdity creates, I believe, demonstrate that the assumption is false: there is in fact no bright line.
Life is not a state, but a process. Human life has many shapes and forms, and is not circumscribed. Taking one aspect of humanity (consciousness, ethical reasoning, ability to feel pain, a heartbeat, etc) and artificially elevating it to The Measure of Humanity creates ethical confusion, because it is an arbitrary choice of many human attributes.
Human life is a nebulous idea. A life-form gradually approximates our idea of human life over a gestation and a lifetime.
So what does this have to do with abortion? I think we need to get comfortable with ambiguity first, than deal with it the same way we deal with any imperfectly knowable body of information. We still raise interest rates and take aspirin, even though we imperfectly understand the workings of the economy and the human body.
Then given what we know about the scientifically measurable features of fetuses, the ethical implications in terms of individual rights and predictable harms, and our inherited body of laws and political institutions, we balance our values for life and dignity to decide how we will exercise our citizenship.
There unfortunately will never be an 'answer' as to whether abortion is right and wrong, or when exceptions apply, with the certainty of a mathematical law. There cannot be, in an issue charged with values, matters of degree, and uncertainty, just as there will never be a final word on something as complex as whether US foreign policy should be isolationist or activist.
There is no absolute answer, only the evolving political/cultural landscape, and we have to decide how we shape it and react to it.
That does not mean that once we make our best guess, we should not be rigorous in convincing others of our way of thinking and exercising our political rights to bring that plan to pass. We should. But we must always have the humility to remember that it is just that--our best guess. That's the essence of centrism.