Well, here's an excerpt from the late Carl Sagan's last book, Billions and Billions : Thoughts on Life and Death at the Brink of the Millennium. In my opinion, Carl Sagan was one of the greatest scientists of our time:
"Despite many claims to the contrary, life does not begin at conception: It is an unbroken chain that stretches back nearly to the origin of the Earth, 4.6 billion years ago. Nor does human life begin at conception: It is an unbroken chain dating back to the origin of our species, hundreds of thousands of years ago. Every human sperm and egg is, beyond the shadow of a doubt, alive. They are not human beings, of course. However, it could be argued that neither is a fertilized egg.
In some animals, an egg develops into a healthy adult without benefit of a sperm cell. But not, so far as we know, among humans. A sperm and an unfertilized egg jointly comprise the full genetic blueprint for a human being. Under certain circumstances, after fertilization, they can develop into a baby. But most fertilized eggs are spontaneously miscarried. Development into a baby is by no means guaranteed. Neither a sperm and egg separately, nor a fertilized egg, is more than a potential baby or a potential adult. So if a sperm and egg are as human as the fertilized egg produced by their union, and if it is murder to destroy a fertilized egg--despite the fact that it's only potentially a baby--why isn't it murder to destroy a sperm or an egg?"
"So, if only a person can be murdered, when does the fetus attain personhood? When its face becomes distinctly human, near the end of the first trimester? When the fetus becomes responsive to stimuli--again, at the end of the first trimester? When it becomes active enough to be felt as quickening, typically in the middle of the second trimester? When the lungs have reached a stage of development sufficient that the fetus might, just conceivably, be able to breathe on its own in the outside air?
The trouble with these particular developmental milestones is not just that they're arbitrary. More troubling is the fact that none of them involves uniquely human characteristics--apart from the superficial matter of facial appearance. All animals respond to stimuli and move of their own volition. Large numbers are able to breathe. But that doesn't stop us from slaughtering them by the billions. Reflexes and motion are not what make us human.
Other animals have advantages over us--in speed, strength, endurance, climbing or burrowing skills, camouflage, sight or smell or hearing, mastery of the air or water. Our one great advantage, the secret of our success, is thought--characteristically human thought. We are able to think things through, imagine events yet to occur, figure things out. That's how we invented agriculture and civilization. Thought is our blessing and our curse, and it makes us who we are.
Thinking occurs, of course, in the brain--principally in the top layers of the convoluted "gray matter" called the cerebral cortex. The roughly 100 billion neurons in the brain constitute the material basis of thought. The neurons are connected to each other, and their linkups play a major role in what we experience as thinking. But large-scale linking up of neurons doesn't begin until the 24th to 27th week of pregnancy--the sixth month."
My argument is that a fetus is not viable (cannot live on its own) until at least the sixth month. As Sagan argues, the fetus does not become distinctly human until then. Until the sixth month, the fetus is a "potential human." This is the school of thought that most scientists and medical doctors subscribe to. You can go to the link on the bottom of this message to read his entire chapter 15 which is devoted to this subject. Pro-choice or anti-choice, everyone should check it out. He talks about the blatant contradictions of a society that slaughters everyday, but insists on the "personhood" of a bunch of cells that are not distinctly human anyway. He also addresses the moral, ethical, and religious aspects of abortion. Actually, Sagan articulates the sexist, racist, and classist nature of banning abortion more succinctly than I ever could. Glad I could present to you a side of the issue you've never seen before.
But let me just say that I know that this debate can go on and on, but I just haven't heard a good argument against abortion that: 1)isn't within a religious context, or 2)addresses the political implications for banning abortion in a effective way that bars sexist, racist, or classist connotations. If you have one, I'd be more than happy to see it. Again I strongly believe that everyone has to right to hold to their own moral convictions. That's why I'm pro-choice.