I think scientifically it is hard to deny that a zygote is indeed "alive". An good example of making this destinction was presented in my biology class. If a carrot is uprooted from the ground and placed in the refrigerator is it dead or alive? Many people claimed it was dead. Why? Based on the same arguments that claim a fetus is not a "living entity " of it's own. A carrot in the fridge is unnable to recieve sunlight, absorb water, or take in nutrients from the ground. A carrot in the fridge is absolutely helpless and unable to take care of itself. It will soon be dead. However, scientifically speaking, it is not yet dead. It maintains living cells which are capable of surviving and reproducing once connection to water, sunlight, and nutrients resume. The cells within the body of the carrot are still functioning and reproducing and as a whole the carrots life functions are still maintaining a state of homeostasis as they were before the carrot was uprooted. There is not a "hybernating stage" where a mass of living cells which is maintaining homeostasis is "unliving".
The same a body of cells within a woman's body is not capable of maintaining it's life once removed from her body. However, it IS maintaining homeostasis from within the womb. Hence it is not really scientifically up for debate whether or not the being in question is alive. The question is of course whether or not it is human and what defines humanity, as I know very few pro-lifers who are indeed "pro-life" in the complete sense of the term. All the plants and animals we consume have in deed been killed. Obviously, if the lives of these creatures are not under the protection of "pro-life" sentiment. Therefore, the only lives which are being advocated for under the banner of "pro-life" are human lives.
Again scientifically, I don't think it is really up for debate whether a zygote is of the genus species homeo sapiens. It is of no other species I know of. I know of no scientists who classify two year old homeo sapiens under a different genus species then 20, 40, or 80 year old homeo sapiens. I know of know definition of humanity that requires that a homeo sapiens have reached full maturity, intellectual development, or spiritual capacity in order to scientifically be classified as homeosapiens. A human being who does not, has not, nor will not ever display any sign of human compassion, morality, spirituality or intelligence will still be considered human if it is genetically created from a homeo sapiens. We do not debate whether or not it is morally acceptable to use intellectually deficient or emotionally retarded humans as a food source, do we? It is, I believe the heart of the matter that as human beings we are able to fully grasp the similarity and therefore we believe that other beings who seems so similar to our selves, must also experience the depth of emotion and feelings that we feels. We do not relate as easily to any other species.
This does not mean that animals other than ourselves do not feel. Rather it means we do not relate enough to care. It means that often we rate a beings feelings not by whether or not that being can feel, but based on whether or not that being can return those feelings and be a use to us or society as a whole. A cow will doubtedly contribute to any human being of it's own free will. There for if it experiences pain upon being turned into roast beef dinner, we don't care. An embryo contributes or displays little more "human" caracteristics than a one month old baby. And a one month old baby is far intellectualy inferior to many species of apes and monkeys. In fact a one month old baby, as far as I know, does not display the ability to reign morally over his/her actions, take resonsability for himself, show devotions to a God or deity, or any of the other signs I hear people porpoting to define a being as "human". I don't see any huge jump in intellectual capacity or spiritual development in a one month old baby from an 8 month old fetus.
Therefore I am convinced neither that a fetus is unhuman, unfeeling, nor unliving. The question becomeshow much can it feel at which stages of develpoment and at which point are those feelings deeper than the feelings of a pig, chicken or cow? Anyone who has seen these animals slaughtered can not really debate that they fight against their own demise the same as small fetuses. And yet no one sheds tears at dinner (unless you are the absurdly rare vegetarian anti-abortion advocate). To me killing anything is always sad. However, if we wish to live to see tomorow, we have to make unfortunate choices. I am not excited about the use of abortion for birth control. However I also believe that as a community we are sorely lacking in support for struggling parents. Adoption is often not the solution that is implied by its' suggestion as a "crises pregnancy solution." Ok have it folks.