What Mr King SnakeBoy is missing and apparently nobody pointed out yet, is that, even if we were to argue that science finds out tomorrow that life starts when that sperm hits the egg, such form of life is entirely parasitical, up until a certain point where it can be extracted and can live on it's own.
That means, that 'life' needs a 'host' for a certain period of time, and it just so happens that the 'host' has as many cons utional rights as this alleged 'life'.
This is about the point where the moral police alarms start sounding, and we get the erroneous 'but the right to life should trump any other right'. Wrong.
Now that we got that out of the way, and the fact that science has do ented pretty well the physical and emotional rollercoaster of pregnancy, we have then established the two competing rights: The privacy right of the host not to have to go through the hosting process, and the 'state interest in life' (which in a way is hilarious, because what would be the state interest outside of the religious/moral motive? The only justification left is 'another taxpayer!', hahaha... erm).
So we don't even need science to get to the 'viability' argument, and the competing interests, etc, etc etc.
The science angle is also interesting, but much more arguable, because science has this third state outside of true or false: we don't know. It's a perfectly reasonable state, but it's a state where you're encouraged to throw all sorts of theories at it and then required to prove them.
So we can argue that DNA = human, or we can argue that DNA is a map of how to build what ultimately will become a human (through the aforementioned hosting process). But unless you can test it and can reproduce the results (prove it, basically), it really has no scientific value, and it's just conjecture.