To help Vixen out in terms of answering the question of when does a clump of cells become a human being, I've shortened and modified a post I made in the other abortion thread:
There is no line that demarcates when a given sperm and a given oocyte become a human being. Certainly not the sperm or oocyte themselves- billions of one and scores of the other are thrown away in one's life time, so each of use leaves a trail of dead pre-humans behind us as we live our lives. And neither biology nor God care- that's the way it is set up!
The moment of conception after that one sperm and one oocyte meet? Well, probably 4 out of 5 of those zygotes are also discarded naturally, often because of severe flaws making successful development impossible, so again neither biology nor a deity appear to have invested much value in that stage. And in fact we do not assign humanity to single diploid cells in any other context: we shed from inside our cheeks every time we brush our teeth, excrete them with our poop. donate them to others when we donate blood, etc. Billions of them die naturally inside our bodies every day. We even intentionally try to kill them as savagely as possible when they become cancers. I have in my lab human cells from people who died many years ago. I use them for medical research to help actual human beings; should I give them funerals when I finish an experiment?
Some anti-abortion people say that a single-celled zygote has the potential to become a whole human being and therefore must be protected! Well I have bad news for them: many cells in our body have the potential to become a whole human being through somatic cell cloning/de-differentiation/somatic stem cell techniques. A cheek cell could become your younger identical twin- should there be a law to prevent you from spitting her or him down the sink?
So when does a zygote become a human life? Think about it: we don't consider killing plants, or fish, or flies murder, so presumably becoming human means the zygote must advance enough to become something special, something characteristic of humans and not of plants or other animals. It is not simply when the heart beats. It is not simply when the first action potentials occur in the tiny clump that will become a brain. Trout have much more advanced hearts and brains than do 3 month old human embryos, yet killing trout falls under fish and game regulations and not the criminal justice system!
Therefore the answer to when a zygote becomes a human being is one that must depends on one's personal views of living things in general and are typically based on considerations of intelligence of the creature under consideration. If you are profoundly sensitive to the sanctity of all life you need to be a strict vegetarian (perhaps one of those who only eat fruit that fall naturally from trees). I would fully respect that. However most of us feel comfortable with eating fish, or even cows; I eat both and biologically speaking the embryo is functionally and mentally inferior to what went into my hamburger.
But okay, perhaps you are a fruitarian and value all life equally. Fine- avoid chicken McNuggets and don't have an abortion yourself. But your views, no one person's views in fact, are so obviously the correct ones as to be imposed by society on all. And in particular, given it is the pregnant woman who carries the mass of cells and for whom the decision is most difficult and most personal, I very much believe it is her decision based on her views. Not yours, not mine, and certainly not the views held by only a minority of people in Ireland.
Taking all this into account, I personally think that drawing the line at which society should have a say is when the fetus becomes independently viable.
