HEll, bruce, if you define your terms right you can make dairy farms look like charnel houses. The assertion that from conception on the individual zygote has full human nature is specious. Folks who choose it as a belilef, in the absence of some better analsyis, are free to live by it.
The real question in my mind is, which legal position brings about the greatest amount of human misery? Having aboirtions causes some misery. Messes up the natural biochemistry and dynamics of motherhood. ABsolutely outlawing abortion causes plenty of misery as well -- those who comply are suddenly shifted into all the cares of parenthood, often without the economic means or skills to do so. Finally, making laws about it one way or another may be the real point of misery, in one sense, because it opens up the whole bag of worms of social law based on half-truth and superstition and opinion. We should refrain from forming laws on these bases.
Most important of all, we should not invade the jurisdiction or presume to arrogate the understanding of the individual whose life is involved in the external world, namely the mother. Badgering women on hypothetical grounds is unwise, for sure.
If we cannot differentiate between potential existence and actual existence as a human being, then we should not pretend to be able to do so. We should not go about forcing people to reproduce, nor prohibiting them from reproducing. This is a core individual right, until such time as a new individual is definitively present. Not just a survivable process.
Brain function might be a good criterion. How many synapses do you want to decree as the dividing point, based on which we will now impose a law on the land, willy-nilly, governing the lives of the sexually active? Choose a power of two.