under assumption naturalism:
--Its tempting to say that until the fetus has a functioning brain, you cannot possibly consider it to be a person...I mean if there have never been any brain waves, how can it have a "personality" mapped to its physical being?
however, even a new born baby lacks any sort of set "brain pattern" linked to something of a personhood above:
Scientists are just now realizing how experiences after birth, rather than something innate, determine the actual wiring of the human brain. "Only 15 years ago," reports the Families and Work Institute in the just-re-leased study "Rethinking the Brain," "neuroscientists assumed that by the time babies are born, the structure of their brains [had been] genetically determined." But by last year researchers knew that was wrong. Instead, early-childhood experiences exert a dramatic and precise impact, physically determining how the intricate neural circuits of the brain are wired (NEWSWEEK, Feb. 19, 1996). Since then they have been learning how those experiences shape the brain's circuits.
At birth, the brain's 100 billion or so neurons form more than 50 trillion connections (synapses). The genes the baby carries-from the egg and sperm that made him-have already determined his brain's basic wiring. They have formed the connections in the brain stem that will make the heart beat and the lungs respire. But that's all. Of a human's 80,000 different genes, fully half are believed to be involved in forming and running the central nervous system. Yet even that doesn't come close to what the brain needs. In the first months of life, the number of synapses will increase 20-fold-to more than 1,000 trillion. There simply are not enough genes in the human species to specify so many connections.
How To Build A Baby's Brain | Newsweek.com
Its not legal to murder brain wave-less vegetables, for good reason. So what do unkillable lower animals have (ie pet dogs), that babies have, that make them "unkillable"? A heart beat comes to mind...however, people have killed mice that aren't in our favor.
Sadly, under the naturalism assumption: killing fetuses or even new borns seems to be wrong if its a) illegal, b) the baby is valued by someone else. until it has a "brain pattern" its just not justifiably intrinsically valuable.
this is an admittedly grisly conclusion. Think how ridiculous it would be though to ever find a baby that was actually valued by no one...
not as possible as you might think. Secondly, would people actually take the time to differentiate and do a brain scan? Where would you quantitatively draw the line as to this "level of brain personality mapping"? Thus, the grisly theoretical conclusion can be tossed out as quickly as im sure everyone wants to (including me!). Drawing the line at the birth point seems to make sense and protect the innocent.
under assumption non-naturalism
--you'd be perfectly right to make up any point within any person's life and say "thats when their soul enters!", and then arbitrarily make that "the point where abortion can only happen before".
--It could be conception, a heartbeat, a brain wave, a first birthday, a communion, 100 years old.
the conclusion is
potentially just as grisly. Again, in practice, its not that grisly. Most religions believe its at conception or sometime during pregnancy.
In conclusion: can anyone really justify the government taking a side based the assumption of metaphysics? would it be a violation of religious freedom? (whose religious assumption of 'the-soul-enters-at-this-point' would the law use?)