Seems we hit the crux of the matter.. semantics and worth of a humans life.
When does a human life start to have some worth? Whatever answer you pick, I can argue something different. And thereby justify any subsequent killing action by the simple virtue that it's not human/person at that point.
I'm sure you can. Just remember that a contradiction is not an argument, it is just the automatic gainsaying of any statement the other person makes.

After all, what makes a human, human? Just bilogy? in that case, a humans' life if worth from day 1.
This is essentially the "value by potential" aspect and if you go by it, logically you should also use it for other purposes or define what makes human worth a special case so it may be used.
Why not use it to establish a value of your car by saying it'll one day be an antique?
Or selling real estates on the premise of surely becoming worth more one day?
Or selling stock of a nonexistant company on the premise than when you actually get the firm up and running it'll be like a goose giving golden eggs? (normally it's called a hoax)
Or saying that once your pants are on, you make gold records?
So why would worth of a human being be defined just by the genotype? Because that's essentially the only thing the embryo and early fetus really have linking them to the developed form of a child; it's the same organism as far as genotype is concerned, but the change is vast yet not clearly defined, as the change occurs gradually.
Which is really what makes the whole issue really difficult.
Brain power? What's so special about that? Heck, every animal has a brain.
Technically no, not every animal has a brain. Actually, not all animals even have nerve cells to begin with. Single-cell animals are of course pretty much given, but there are also multicellular organisms like that - sponges come to mind for example. Then there are animals that have nerve cells but no brain; jellyfish, starfish, hydra, sea anemones and the like. They have a decentralized nervous system instead.
All vertebrates do have brains though, and most invertebrates that most people think of as animals (insects, crustaceans, arachnids, cephalopods etc.) so while it is incorrect to say every animal has a brain, it's usually a valid statement in everyday speech... but incorrect it is nevertheless.
And when does a brain become big enough and complex enough for the human life to have worth?
While size of the brain typically doesn't correlate to mental capacity, it still needs all it's parts functional to some degree to be able to function normally (yeah, the brain can compensate for damaged regions relatively well, but you still need a functional brain cortex for consciousness and voluntary movement). And of all the organs of the fetus, the brain developement doesn't stop during the pregnancy, while other organs are pretty much ready after the second trimester (if I recall my biology lessons correctly). Lungs only start working properly when the baby is big and strong enough to use them physically, though.
I don't know at which point the brain cortex reaches a point where the fetus becomes aware of sensory input, or when it becomes more aware of itself, if at all.
Personality?Newborns don't have a personality yet. They are blank slates yet to written upon. No worth whatsoever then.
Eh?
Why would they suddenly start to develope personality only after birth? They do have all their mental and cognitive abilities for quite a while before they are born, it's not like they magically start from nothingness after birth. They have a lot of sensory input and, while their motoric range is kinda limited, there's no reason to assume they do nothing while in utero. They have periods of sleeping and activity much like after birth. The only things that are introduced in birth as a transition are:
-individual digestion
-individual breathing
-exposure to elements (temperature is no longer constant, the immune system needs to start working out etc. etc.)
-direct contact to and interaction with people and external world
I'm sure you agree that for the developement of the child's psyche, the last one is the most important change that occurs in birth, but does it mean that the child can have no personality without it, even in utero?
I don't think so. Besides, ask any nurses or mothers if newborns have a personality or not... and most would answer that they definitely do.
Inherent empathy? Some humans are cold as blocks of ice.
Are you referring to sociopaths? Because that's a mental/personality disorder (although some would just disregard the psychiatric diagnosis and call them arseholes or something less flattering) or a few of them at the same time. It's an illness of the mind, and even if it might be genetic, I don't think they are incapable of knowing empathy, it just doesn't make sense to them. Many of these people can act like normal people if they so choose, but to them, other people don't have any value. That doesn't mean other people should think of them as worthless, obviously.
Aside from these individuals that are quite obviously an exception of normal human behaviour (yet somehow evolution has not deemed fit to remove them from the gene pool; clearly these personality types are beneficial in some situations), empathy is a core feature of typical human behaviour, but you're right - it can not exclusively be used to define human behaviour, much less human value.
Usefulness to the species? Chuck the old and impotent into the fire then.
Blarrg, it seems you really slipped on that surface. Or are you just throwing out straw men? Of course that's not a valid criterion for human value.
You can start applying labels and categories, but there's really no difference in saying a fetus isn't a person, and therefore can be killed, and saying that mass murdered isn't a person, and then frying him on a electric chair. It's all down to the criteria and semantics and applying worth.
It is not the same thing because the mass murderer quite obviously is a person. He walks and talks and does stuff, while the existence of a fetus (without the movements) would, up to certain point, much comparable to someone in persistent vegetative state.
Frankly, this now dwells deep into the philosphical aspect, and whole books could be written on any of those questions.
True.
If it weren't difficult and ambiguous matter, this kind of debates wouldn't be going on.
I don't like the concept of late abortions either. Aborting a pregnancy at 24th week when the fetus could potentially live after birth at week 21 or 22 (albeit with low chances of survival) creeps me out.
Which is why it's a good thing that the vast majority of abortions occur in the first trimester, as far as I know.
But, like I said "human value" is not clearly defined in itself, and I would personally grant it to the other great ape species (chimpanzees, gorillas and orangutangs) with no question. Cetaceans likely as well, though they are a bit more difficult to deal with.
By the way, how does using a chimpanzee as a test subject for a lethal procedure compare with early abortion in your opinion? Worse, less aggravating or equal?