As a doctor who has delivered thousands of babies Ron Paul understands abortion on a medical level better than most politicians. And for this reason Paul is against abortion. But he's not a pro-lifer in the modern-Republican sense either... Instead he believes it is a state issue. He would leave it to the citizens of each state to decide for themselves to what degree and under what circumstances abortion should be practiced.
This is yet another reason why Paul is highly electable. He's not a flip-flopper who panders to the Right or Left. He just uses principled common-sense judgement to create responsible policies.