Most people like to call themselves pro-life for a whole slew of reasons. My reasons are my own and for most people, I do try to keep religion out of this even though I am a devout Catholic.
I am of the opinion that a cell is alive. What makes it human, I think, is its capacity to grow normally into a mature human being. That is why skin cells are not human life, because they will not grow into a mature human being. But what does this say about the embryo? This means that I do consider it to be alive because aside from its age, it is the same as you or me.
A lot of people like to think of it as just a mass of tissue connected to the Woman's body. Some even think that to be pregnant is punishment. Both of these views are irrational. Becoming pregnant in any way is not a punishment: it is a biological function for reproduction. Yes yes, it is a beautiful gift from God, but we are not looking at this from a religious perspective. As a biological function it makes sense that to propagate the species, the child must be allowed to live. Survival of the fittest and all that. It is observable that no animal willingly kills its unborn unless it is forced to (pregnant monkey on hot plate experiments). In fact its first instinct is to protect that still unborn at its own risk. Therefore it is against all biological reasons to have an abortion or even contraception.