"What gets eaten, what gets aborted, what gets smashed, will never be decisively resolved by an inspection of internal structures and capacities of a given candidate for destruction. Even smashing a mere chunk of solidified lava -- evidently purely passive, and homoeomerous from one end to the other -- can be experienced as a transgression by the person who is properly sensitized, for whom the chunk shows up as salient within her ethically charged environment. Are fetuses morally relevant? Yes, they are. So are chunks of lava. Does that mean you mustn't destroy them? Not necessarily, but you shouldn't suppose that the way to gain license to destroy them, whether this license is conceived cosmically, socially, or individually, is to produce arguments that cut them off from the sphere of moral relevance."
Justin Erik Halldór Smith, The Moral Status of Rocks. An excellent post, worth reading in full.