I understand your argument, but where do you draw the line regarding "sentience"? Is it your only criterion? Suppose I have a baby with anencephaly, or a person in a persistent vegetative state after a car crash. Can we simply abort them because they are no longer sentient? We had a professor talk about the problem of the main criterion why abortion should be legal/illegal. He told us the story of a scholar who proposed "independent living" as the criterion, which would enable abortions up to the age of six years (and of course, lifelong abortion for anyone not capable of independent living such as mentally or physically disabled persons). I'm totally on your side that women should be able to legally abort, but to date I have not seen one philosophically sound argument from pro-choicers.