Carrier: From a point of view outside of this affair, the killing of a neurologically inactive fetus is no greater a harm than the killing of a mouse, and in fact decidedly less--a mouse is neurologically active, and though it lacks a complex cerebral cortex, it has a brain of suitable complexity to perceive pain (and I would argue that the mouse deserves some moral consideration, though less than humans). A fetus cannot perceive pain (and perception is not quite the same thing as sensation: sensation can exist without a brain, but perception cannot). The neural structures necessary to register and record sensations of pain transmitted by the appropriate nerves either do not exist or are not functioning before the fifth month of gestation. A fetus can no more feel pain than a surgical patient under general anasthesia, or a paraplegic whose lower-body nerves continue reacting to stimuli, but cease sending signals to the brain. And we have already established that a fetus does not contain an individual human personality of any kind, any more than a brain-dead adult does. With no perception of pain, and no loss of an individual personality, the act of abortion causes no immediate harm.
This is a line of argument often used on the pro-choice side of the debate. Bonnie Steinbock uses this line of argument in her case against Don Marquis' anti-abortion paper.