An extremely difficult question. And one that causes a wee bit of a fuss occasionally, or so I'm told.
Life begins at conception, say some. But that doesn't necessarily mean it's automatically sacred. It's an unfortunate biological fact that conception happens quite frequently--but the resulting (is it zygote?) doesn't always attach to the uterine wall. Frankly, human lives are getting pissed out all the time, through nobody's fault, even if there are no birth control pills involved. So if you think there are millions of extra baby angels the size of a pinhead up there, feel free. Blame God and his mechanical ineptitude.
The more complicated and intelligent the life form, the more it deserves protection. Most people don't have issues with eating chicken, but would balk at killing dolphins. They're too smart. It's creepy. What's the cutoff point? It's hard to draw an arbitrary line, and say "this life form is intelligent, this one is not". And when you're talking about abortion, you have to decide. Is a group of sixteen cells a baby?
To determine the rightness or wrongness of abortion, you have to first determine the underlying questions:
1. When does life actually begin? You'll have to define "life" to start with, then figure out how many cells you have to have to count as a living entity.
2. If it's okay to take life below a certain level of intelligence, what's the cutoff point? Start by defining intelligence.
Real easy to figure this out, eh? Most people seem to either go with:
The Catholic position that life begins at conception, all abortion is automatically wrong. This position is at least consistent, and dodges the second question entirely. (It also fits into the Church's position against the pill, because the pill discourages fertilized eggs from sticking around.)
Or,
Dodge both questions and leave it up to the mother/potential mother to decide, because she's the one most immediately concerned. If there is a concern about protecting (potential) people under the banner of common humanity, it is made secondary to the mother's rights over her own body. Conflict between the two is troublesome, but the idea of legal intervention to force an unwilling woman to carry a baby is more repellent than the idea of abortion. Compromise is made by creating cutoff dates --three months is okay, four months is not okay, etc. The problem with this position is that it hasn't really answered either of the two questions above, and therefore has inherent inconsistencies that make the position philosophically weak. That doesn't mean the position is wrong, just that it's more difficult to argue.
Personally, I dislike the idea of abortion. I dislike the idea that people aren't allowed to control their own bodies. These are incompatible in an abortion debate. So my position is that the ethical thing to do is prevent unwanted pregnancies at all costs. Making birth control widely and cheaply available, improving sex education so people realize the consequences (and none of this abstinence only crap), and vastly increasing the ease and availability of adoption would at least cut down on the number of abortions. I think it should stay legal, but the women concerned should be given all the other options. But no condemnation should they choose abortion, either. I can't imagine it's an easy choice in any circumstances, and I feel sorry for people faced with it.
Sorry for the length of this. As usual in murky questions, I see validity in both sides, and my attempts to reconcile them lead to muddled abstraction. Also, since I'm male and not planning to impregnate anyone, I sort of occupy the sidelines on conception issues, and am therefore less emotionally attached to either position.