(Hey, I've seen worse titles!)
Note: The following is adapted and expanded a little comment I made in a blog of Mario Lopez . Much of what follows has been said in other posts recently here, but not exactly in these words ... and not everything is repeated.
Also, try to answer finally the question of Samuel: "What is a human being? When does a fetus become a human being? ", Although the post-comment-turned, in general, was not written to answer this question, but that the post of Mario Lopez. Hence it is expanded, and not copied. 
Without Further ado ...
I think it's the story: the separation of morality and suffering.
If we define "morality" as that which reduces suffering, then the legalization of abortion is obviously moral reduces the suffering of mothers and children does not condemn "forced to be born" (for a state that does not have nor should have nothing to do with it) the lives of suffering.
The problem is when we change the definition of "morality", so this may not be minimally related to the suffering or the reduction / increase this. This is the case: who is against abortion under any circumstance have no problem condemning millions of people to lifetimes of suffering. They see themselves as "pro-lifers," and it makes them feel very well. 
It is what gives becoming overwhelmed by emotions, and not think about things ...
What defines a human being? If you're religious, it is undetectable and undefinable something called "soul." It is not my case, obviously. So which is it? I think it's the brain. A fetus has no brain formed .. It is a living being? No doubt ... after all, any of the cells of the body is alive, and many die and are born every day. But it is a human being? I believe that it is not yet ... but even this may be debatable, what is not debatable is that the mother is ... and is also owner of his body.
Anyway, for me the question of suffering is the most important. All developments of morality throughout history (de-legalization (not necessarily ban it, but these things were part of the law in time) of racism, slavery, torture, inquisition of religious women as property - some of which yet to take place in Muslim countries) happened because people, over time, realized that an abstract and arbitrary morality did not justify the unjust and cruel suffering of millions.
That is why, today, who says he is in favor of slavery will be seen (and rightly) as a monster, an abuser of other human beings, a fossil of many others who, at times, have made millions for centuries and centuries. His position is one that was accepted by most, centuries ago. Considered not only acceptable, but moral. But the morality of the people changed, fortunately. Evolved.
The criminalization of abortion is just one of those cases. And one day, will look exactly the same way.
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