After watching this debate on abortion I think that the point at which life, and a person's rights, begins is an unclear issue.
If you insist on having even a limited government, then one of the laws should be that no one is allowed to harm another. Therefore, deciding when a person "begins" is the point at which his right to not be harmed begins.
A point I liked from the above linked debate is that the point when the unborn can survive without the mother could be important, because that's the point when the unborn stop living at the expense of another.
However, that's an awkward point becasue science changes all the time and abortion laws cannot be changed that frequently.
As far as I can tell the logical abortion law options are:
1. Life begins at conception. No post-conception birth control. Abortions are illegal.
2. Life begins at birth. Abortion is legal until part of the unborn exits the mother. Abortions are legal, partial-birth abortions are not.
I like both options, for different reasons. The imposition on the mother could be considered as the unborn "harming" the mother, but the right to live should be given priority over discomfort and pain.