Let me start out by saying that I'm completely pro-choice. The reason for this post isn't to pontificate about my personal "oughts" on the subject, but rather to lay out why I'm pro-choice. This is hopefully lightly written, but the subject itself requires a warning that you might not want to read further if you're sensitive (I don't like reading about this stuff myself sometimes).
1. Bodily Autonomy
A body is a body, and is owned by a person who is an emergent property of that body. Other bodies can't use your body without your consent, full stop. The rest of the below reasons aren't really needed, since this one is all you need. If you disagree, the authorities will be by shortly to remove one of your eyes and one of your kidneys (c'mon, you'll still have one good one and somebody who needs one will get it).
2. Biology is Messy
Not messy in the "yuck" way, but in the way that every physical body is different and that no two situations are ever completely the same. Any attempt at legislating specific bounds to abortion is just going to complicate, confuse and ultimately restrict the patient-doctor options. Medical decisions should be made only between the doctor and the patient.
3. Sex != Babies
Sure, sex can result in babies (insert naturalistic fallacy here), but babies are not a necessary result of sex. If you think it is, then you need start legislating baby quotas based on copulation rates. Until then, sex is for more than just for making babies. If you're not having sex to make a baby, you aren't required to have that baby should you get pregnant.
4. Reproductive Control
People who want to have babies, now or later, also need to have the choice. I'm talking about people who responsibly use birth control prophylactics for the purposes of #3 above. I'm talking about people who are actively trying to get pregnant and run into the reality of #2 above. Even if you're doing everything right, you might find yourself in the situation where you will need to choose.
5. Nasty Crap
Until the world is a perfect place for women, a world where there is absolutely no sexual violation, there should be no restriction on abortion. The anti-choice mob like to mock and minimize these situations, but as much as they try, these situations are real and do happen. The last thing that women in these situations need is a bureaucratic and legal jungle between themselves, their doctor and their choice.