Mike Almeida has an interesting argument against abortion that assumes nothing about the moral status of the fetus. It relies on two commonsense claims:
1. We should remove a benign tumor that will eventually become malignant.
2. If we should remove something that's not yet harmful because it will become harmful, then other things being equal we should not remove something not yet good that will become good.
Some will surely resist the second claim, which is what the parallel reasoning relies on. But it does seem to me to be a generally true principle. It's why we shouldn't pull up flowers before they finish growing.