It seems to me to be more a matter of balancing the rights of the mother with the rights of the child, and in the process respecting that other people have different opinions than us. This is where the pro-life stance fails: It wants to impose it's view on others. The pro-choice stance, on the other hand, is not, you'll notice, a pro-abortion stance. Pro-choice is about giving the person a choice. If they do not want an abortion, then they don't have to have one.
That being said, how do we decide personally, and legally, where to draw the lines?
We have to admit that things die, and we kill them. I've killed innumerable plants, countless mosquitoes, and even some small animals. Do the plants matter less than the mosquitoes or the small animals? I don't logically think so, but emotionally I do. Further, as a practical matter I can hardly avoid killing something now and then, even if it is only the bacteria in my mouth that the beer I just drank killed. Also of note is that legally, and socially to an extent, it is fine to (humanely) kill almost every other animal besides humans. I won't go to jail for killing that squirrel in my yard; Not many people will mind if I shoot a deer to eat.
So where does that leave us?
When an egg is first fertilized, it is very small. If it wasn't a human egg, no one would care (or even know) that it was 'killed'. It is also almost indistinguishable from an unfertilized egg. I think it is safe to say that a freshly fertilized egg is not deserving of legal or emotional rights.
On the other end of the spectrum, a newborn baby is practically and emotionally 100% a human person, in the same way that a small tree is no less a tree than a big tree. We as humans have clearly delineated that babies are equivalent to adults in rights, whether they be human or not. Killing a endangered protected wild bear baby is just as illegal as killing an adult - possibly even more emotionally offensive. So clearly, once out of the womb, they should not be killed.
The question we are left with is, at what point do we draw the line?
Currently, laws state that a 'person' gains rights at birth. This is the point they gain Independence, and this is the point that we can all agree that they are a human by any definition. To go before birth starts to create some very tricky problems. Even a day before birth, the child is dependant on the mother, and any action to remove dependence is extremely invasive to the mother (c-section, birth). Any line you draw before birth is going to be hotly debated, and not all will agree upon it. Clearly then, the fairest, safest place to legally draw the line is exactly where it is at (in Canada, where I live, you can have an abortion up until birth). Personally however, the line you draw is going to be arbitrary. You are killing something, but we do that every day. It has human genes, but so does the skin cell I just killed as I itched my head. It is going to depend on what you value - the beginning of the heartbeat; The start of mental activity; The first tangible kick. I don't know where you will draw the line, but I am content to let you decide that, within the clearly delineated lines that we as society have agreed on.
