Another question is whether an unwelcome fetus should be considered to be an unauthorized trespasser in its mother's body. The non-aggression principle does not protect trespassers from the owners of the property on which they are trespassing. It can also be argued that unwelcome fetuses are themselves committing aggression against their mothers by taking materials (oxygen, water and nutrients) from her bloodstream, injecting toxic metabolic end-products (carbon dioxide and creatinine) into her bloodstream and preparing to subject her to major medical/surgical trauma in the form of full-term labor and delivery.